Here is a screenshot from the Wrath beta that includes the UI.
Compare it to the very oldest screenshot that includes the UI I was able to find in my screenshots folder, from one of WoW's stress tests pre-beta (summerish 2004):
Looks a bit more crowded, no? Let's go over the thing quadrant by quadrant, Wrath image first, and then WoW beta.
Top Left (Health Bars):
Alright, not much new here other than the numbers on the bar (which also can appear on your target, and target of target; I don't think target of target was an option at launch, and I'm sure the numbers weren't.
Top Right (Map):
Alright, so buff durations are good, as is the minimap tracking selector that went live in patch 2.3 (the magnifying glass in the top image). The blue B doesn't count (it's the beta feedback UI), the little round brown button brings up in-game map (useful), and the large button on the top right brings up the new Wrath in-game event calendar. The big picture of the Sun in its place in the beta image indicates whether it is in-game day or night (like the lighting doesn't make that obvious), and could be moused over to reveal the server time (which is now displayed on the brand new clock, added in patch 2.4.3 a few weeks ago).
Bottom Left (Action Bars):
Alright, here's one thing the "modern" interface improved on drastically; more action bars. I suppose you could bind keys to everything, but all classes need more than 12 buttons to function, and remembering shortcuts gets difficult for less-frequently used abilities (which are most likely to be off the bar). The other additions are the green rep bar (which I suppose is useful since rep grinds are such a common activity at endgame, though I'd rather have a Fubar plugin), and an indicator of which of the six "pages" of action bar you're looking at.
Bottom Right (Options menu):
First off, more action bars (there's a second vertical one next to the Netherwhelp). The number on the backpack indicates how many empty slots I have in my bags, and the key next to it is the keyring, holder of various keys such that they don't take real bagslots. The green bar next to the backpacks in the bottom shot is a latency meter; it has been removed from the modern UI, but the info is available by mousing over one of the other icons (I think the computer?).
And now for the options menus. The portrait is still your character sheet (but more on this in a minute), the book is still the spellbook, and the bullseye is the talent panel (missing from the beta screenshot because Druids did not have talents yet at the time, but it was present by launch). I cannot for the life of me remember what the heck the purple thing on the oldschool menu does (anyone know?). The shield on the Wrath UI brings up the new Achievement menu. The cup is still the quest log, the speech bubble is still the social panel. In the Wrath UI, there's an Alliance crest next to the social button (presumably a Horde one for Horde characters), which brings up the honor page (more on this as well in a minute). If I recall, the globe on the oldschool UI summons the World Map (moved up onto the minimap in the modern UI), while the green thing on the modern UI brings up the (generally unused) LFG interface. The last two buttons bring up game options and the help screen/GM tickets respectively.
I don't have a screenshot of the ancient character panel, but I'll settle for one last shot of the new Wrath beta pet/mount UI.
Players who do not have pets generally do not have a pet tab on their character sheets on the modern interface. (The pet tab has three subtabs, one for your actual pet, if any, one for non-combat pets, shown here, and an identical one for your mounts.) The fifth slot is currently reserved for the PVP tab, but it got bumped to the increasingly crowded options menu to make room for the new tokens UI (which is not yet implemented). If it were up to me, I would have attached tokens to inventory somehow rather than characters, and one could argue whether characters who have neither pets (combat or otherwise) nor a wide variety of mounts really need to have a tab on the character panel (which brand new players need to interact with from day 1). Perhaps it would have made sense to stick the mount/pet UI on the NPC Stable Masters found at every single settlement in the game for the benefit of our Hunter friends?
Let's be clear, all of these options are useful to me. I've also been playing this game for four years now. Part of the increased number of features is convenience, and access to stuff players wanted to know (and coded mods for) from the beginning. Part of it is about tracking the increasingly complicated progression of the game (reputations, PVP/tokens, pets/mounts, achievements, etc). Others are optional (though, again, it's hard to play the game without some source of additional action bars. I'm not sure that there are very many, if any, of the features that have been added that are bad on their own. That said, I can't help but look at the modern UI and wonder whether it has gotten too crowded.
Perhaps your character sheet could go back to being a character sheet, and we could have a spellbook like interface that offers info on your pets, mounts, rep, PVP, talents, tokens, achievements, etc? Perhaps we can integrate the calendar, social panel, and LFG interface? Don't get me wrong, all this stuff needs to be accessible somehow, but the current interface is so complicated that it is actually hard to explain to a beginner (see how long it took me to explain just the buttons that are visible by default, to say nothing of the contents of your spellbook/quest log etc?). I'm not a GUI designer, so there are probably far better ideas than these, but I can't help but look at the interface in WoW today and wonder if it could be streamlined somehow. Ultimately, my solution is to run Bartender and hide half of this stuff, only activating the bag or options menus when they're needed, and that's a pretty drastic measure to have to take.