When the spec required to play Wrath was announced I know a few people with older computers groaned a little, but thankfully I upgraded my machine about a year ago so its still fairly solid for playing wow. The reason for the higher PC specs wasn't immediately apparent upon entering Northrend (though the world as a whole looked prettier) but quickly became apparent: just as the polygon count of characters increased somewhat for the Draenai and Blood Elves in Burning Crusade, so the item detail has increased considerably in wrath. On Andro it's not immediately apparent as her only visible Wrath gear is her shoulderpads, but on my dwarf Paladin Buliwyf the distinction is pretty apparent.
If you look on the picture to the right, Buli's shoulders and helmet are Wrath kit, rendered in high detail. The rest of his gear is BC blues ... and the lack of detail is readily apparent. Compare the boots and gloves to the shoulders and hat (and even the face and beard for that matter). Soon all characters will look that good :)After a slight misshap involving dragonkin, a doppleganger and an hourglass I also noticed another graphical feature: usually when you're dead your kit is uniformly translucent. In the picture on the left, you'll notice that Andro's shoulderpads are a little more obvious ... and they're the one visible bit of Wrath kit she's wearing (from a Nexus quest). I'd be willing to bet (though not to pay the repair bills to prove it) that if her velet cowl (from Utgarde Keep) was set to be visible it'd appear more solid too. (EDIT: Llanion points out below that the shoulders issue is an old BC issue and not something linked to the higher-rez kit).
The other thingh I noticed (as Andro) is the quality of the sound. The WotLK music is great (as I've come to expect with Wow) but the ambient sound is good too. I really noticed it while zapping things uderwaterl the bass-reverb of spells going off. This may well not have been a new feature of Wrath but could've appeared in a previous patch and only become apparent when I was fiddling with sound settings this last week (particularly "headphone mode") but it was doing the new content where I noticed it.