On the off chance that you haven't noticed, the Interwebs are ablaze at the moment, with the release of several new features to Wowhead. OK, so maybe "ablaze" isn't the right word. Perhaps "smouldering." Possibly even "smoking a bit, but not an actual fire hazard per-se."
The feature that most people are yammering on about are item scales. If you don't know what they are, these are basically a scoring system for gear. You assign relative scores to various attributes and bonus effects, and Wowhead will then compute relative scores for different bits of gear. For example, let's say I'm a very narrow-minded fire mage who earnestly believes that every point of +spell damage is worth 10 +intellect, and that's all there is to gearing.
Here's what the Wowhead scale would look like:
And here's a snippet of how it would score some items:
As you can see, this is pretty useful for getting a rough idea of how valuable a piece of gear is. You'll also notice that since I did this search on "Armor" in general, it's not limited to stuff Mages can actually wear. The scales can be used in conjunction with the existing filters, so you could easily search on gear usable by Mages that drops in heroic-mode instances and is epic, or whatever you want.
However, item scales aren't new; they've been around for ages. The first (at least, that I know of) was AEP or Agility Equivalence Points. It was designed for rogues to give them a simple way to rate gear. There have been a few other point systems dreamed up over the years (TankPoints is one notable exception in that, as far as I know, it actually does some theorycrafting on the fly for you as opposed to simply summing attributes.)
But what's really got me excited is how well the new Wowhead scales work with an old addon I've been using for ages: Pawn.
This addon basically does exactly the same thing that Wowhead now does: you tell it the relative worth of different attributes and bonus effects, and it will compute a score for each piece of gear you look at. The great thing is that instead of having to fire up a browser to check an item's score, Pawn will insert the score into the item's tooltip.
The big problem with scales is coming up with them in the first place. Take mages as an example. How much is crit worth relative to spell damage? Relative to intellect? Relative to strength? 
However, the fine, fine human beings at Wowhead have done most of the work for us by coming up with default scales for every class and spec. That's not to say they're perfect; but they do give us a starting point. What's more, now that this exists in the main stream, it's likely we'll see people posting "optimised" item scales that we can use.
Let's say you've found an item scale that you like. Like, say, our fire mage scale above. You'd rather like to be able to use that in-game. Well, OK; maybe not you personally, but work with me here. This is long enough without trying to do something actually useful.
First of all, grab Pawn and install it.
Once you've done that, you can bring up the Pawn configuration by typing /pawn. There's also a button on your character sheet if you'd rather use that. You'll get an initial "go read the documentation," message, but we can ignore that for now. Click on the "Options" tab down the bottom.
A few notes on these options.
Show item levels will show an item's level in its tooltip. This isn't the "required level;" an item's level determines how much of each stat it gets. For example, weapons that drop off heroic end-bosses are ilvl 105, whilst gear from those same bosses are ilvl 110; Tier 4 is ilvl 120, except for stuff that drops off Prince Malchezaar which is ilvl 125. That said, the item level is not the be-all and end-all.
Show item IDs will tell you an item's unique identifier. For instance, my [Surestrike Goggles v2.0] have an itemid of 32474. Fun fact: look at the URL for Wowhead's page on that item; the last part of the URL is the itemid itself. This means you can always get an itemid from an item's tooltip in-game, and then directly enter the Wowhead URL for that item!
Show inventory icons displays the icon for an item above its tooltip in some cases. Personally, I prefer it off, but turn it on and see if you like it.
Add a blank line before values does exactly what it says: it adds some padding into the tooltip.
Align values to the right edge of tooltip also does what it says; with this turned off, the scores are right next to the scale's name; this makes them a little harder to find when you're scanning a tooltip, so I prefer to have this checked.
Show (*) on unrecognized stats relates to things that Pawn doesn't know how to score (for example: the minor run speed increase on some meta gems.) If it finds something it doesn't understand, it puts "(*)" next to the score. I turn the warning text off because I'm well aware of what it means by now.
Digits of precision is useful if you have a lot of really close scores. Personally, I leave this at 1 and try to engineer my scales so that scores aren't so close together that I need to increase the precision.
Calculate unenchanted values will display an item's score as if it wasn't enchanted. This is important because when you get a new drop it's not going to be enchanted, is it? This levels the playing field a bit.
Calculate enchanted values will display an item's score including its enchant. This is important because when you get a new drop it's not going to be enchanted, is it? This lets you know if the new drop is already better than what you have even before enchanting.
Normalize values can be useful if your scores are all 6+ digits; your scores will generally drop to very small numbers with this. It doesn't change how they're computed, however; it's just visual.
The rest you can leave or fiddle with, but they're the important ones. So, with that done, let's look at porting a scale from Wowhead to Pawn, shall we? Click on the "Scales" tab. You should see the default Pawn scale; uncheck the "Show this scale in tooltips" box down the bottom to hide it. Now, click on the "New empty" button, set "Fire Mage" as the name and hit enter.
Now, let's get to work. The big list on the left contains all the things Pawn knows about. When you click on one, a description shows up on the right, along with a box for entering its value. First of all, we'll click on Intellect, and set it to "1."
Next, scroll down the list to the "Spell stats" section, and select "Spell damage." Set that to "10."
That's it; we're done! Because we're a Fire Mage (not really,) change the colour to a nice firey orange and click "Close." Now, let's see if it's working...
Cool, huh? Note that it's not the same score as Wowhead gives us, however. This is because Wowhead and Pawn use different normalisation techniques for the scores. If you go back to the options screen and check "Normalize values," this will get the two close, but they still won't match.
So if they don't match, why is this useful? Well, imagine you're in an instance and some inconsiderate bastard starts looting corpses while you're still busy DPSing your little heart out. The item pops up and... ooh, that might be useful. But is it better than what you've got? Hmm, not as much spell damage, but it does have a bit more crit...
In situations like these, you can look at the scores in the tooltip to give you a quick indication of whether a piece of gear is worth further investigation or not. Combined with an addon that shows comparison tooltips, it can save you a lot of time better spent causing things to violently explode.
Now, there are a few more caveats: Wowhead's system will automatically fill gem slots with whatever gems give it the best result (maybe; it's not documented anywhere, but that's what it looks like it does.) Pawn doesn't do that; instead, it allows you to assign a score to different coloured sockets. So what to do?
The trick here is to look up the gems you're likely to put in those slots. Count up its stats, and work out what the gem's score would be. It's important that you don't just look at Wowhead's score for the gem, since this won't be directly compatible with your Pawn scales. It's not that hard, though. Take [Runed Living Ruby] as an example. We said that each point of spell damage was worth "10" in our scale, so the gem itself is worth "90." Assuming you intend to put those into your red gem slots, plug in "90" as the value for red gem slots.
Meta gems are a complete pain in the rear end, but since you're not likely to ever upgrade to a piece of stylish scalp-protector without one (at least, not until WotLK,) once you're in gear that has them, you can probably just ignore the meta gem value (or give them some silly high value like 1000.)
There's another trick that might be of interest to the hunters out there. You all know how weapon speed is basically the deciding factor in ranged weapons, right? Well, Pawn supports a nifty little trick that Wowhead doesn't.
First of all, you need to decide which direction you want to go with weapon speed. Head down to the "Special weapon stats" section, and select "Ranged: speed". If you're BM, enter a big negative number (which will give higher scores to faster weapons;) otherwise enter a positive number (which will favour slower weapons.) This part will also sort of work with Wowhead. 
What you do then is scroll right down to the bottom and select "Speed baseline" and enter your ideal weapon speed. In my case, I set "Ranged: speed" to -500, and "Speed baseline" to 2.5. This means that weapons faster than 2.5 will gain points, and weapons slower than 2.5 will lose points. Nifty, eh?
One last final tip before I go: you might not be happy with the scores. Specifically, if you don't normalise, you can end up with big scores like 3890, and if you do normalise you end up with tiny scores like 1.57. If you want something in the middle, you can divide all the attribute values by a power of 10. So instead of using "68" as a score, use "6.8" or ".68". Just remember to do this to all of your values.
And that's really all there is to using Pawn. I would recommend that you use as many scales as you have roles. For instance, Druids should have a "Tank" scale, a "Kitteh DPS" scale, a "HPS" scale and a "HPM" scale. This will let you quickly decide if any particular piece of gear is an upgrade for one of your gear sets or not.
Does this mean you can rely entirely on Wowhead and Pawn for gear choices? Don't talk crazy like that. Things like this aren't a substitute for experience (or at least, really complex theorycrafting,) but they do help you make quick "ballpark" guesstimates.
P.S. If you didn't get the joke in the comic, that's because you're pronouncing "pawn" and "porn" wrong. No, the British pronunciation was in use first, ergo it wins.
 Yeah, OK, that last one was a trick question. :D
 "Sort of" because Wowhead seems to have hiccups with negative scores.
Image of tasty looking tempura prawns © Swamibu, the not-real-at-all girl who is actually really wearing a bikini under those black bars © myvirtuallady, and the pawn that I spent like 10 minutes cutting out is © Jud McCranie.