Here's a list of various reasons why in my opinion Wotlk > Classic!
New game content such as dungs/bgs/raids/new areas/new classes/new races etc obviously aren't included, because those are large pieces of content. I am focused here on small things... there's a lot of them though and they add up to create an overwhelmingly more polished game. That's why i prefer Wotlk over Classic for an example. Classic was great though aswell, don't get me wrong :)
This list might serve someone who's thinking about either playing on a Wotlk realm or a Classic one. I'm sure there's even more things to list, add them in the comments if you'd like :p Maybe this will make ppl appreciate Wotlk more aswell, because a lot of you might not even know how the game changed.
What Wotlk has and Classic doesn't basically:
1 class colors in overhead nameplates
2 limitless debuffs on mobs, instead of 8 or later 16 etc
3 you can swim with your mount
4 the underwater breath bar lasts way longer
5 you can get a mount at lvl 20 instead of 40 and an epic mount at 40 instead of 60
6 riding skill costs less
7 you can start casting a spell from sitting down, you had to stand up before casting in classic
8 you can start casting a spell from being mounted, you had to dismount before casting in classic
9 dual spec
10 focus frame
11 you can have 25 quests instead of 20
12 you can track more than 5 quests simultaneously
13 hearthstone cd is 30 min not an hour
14 fewer skills need reagents
15 fishing cast time is faster
16 way better addons
17 party leader's text has a different color so it's easier to distinguish the person in chat
18 more bank space
19 more mailboxes
20 more graveyards
21 faster running while dead
22 more flightpaths
23 you can see the duration of the debuffs on an enemy
24 quests can be seen on the map
25 minimap has the tracker, with which you can trak various things on the minimap
26 stopwatch with various time settings
27 calendar
28 minimap shows the date
29 possible to collapse the quest tracker on your UI via the small arrow and also make it reappear
30 a smaller version of the map (i don't mean the battle map "shift+M" one)
31 can queue bgs from anywhere
32 when you hover over an item, it shows the sell price of it
33 when you hover over an item and press shift, you get a comparison to what you have equipped currently
34 equipment manager for instantly switching on an entire different set of gear or parts of it
35 talent preview
36 a tab for pets/mounts, instead of them taking up bag space
37 achievements
38 currency tab
39 the mini menus of various stats in your character window
40 minimap clock
41 professions are much easier to learn, because you don't have to go to all kinds of places for almost each rank of each profession
42 the guild name of a person is shown when you over over them, in the lower right corner of the screen... in classic the guild name was not shown
43 possible to link professions into chat
44 possible to link quests into chat... yeah you could do that before too, but it only provided a dead link, other's couldn't click on it and it didn't have color that depends on the person's lvl who is viewing it
45 hovering the mouse on a mob shows all the quests that this mob is relevant for and shows the progress too
46 you can see quest givers on the minimap
47 enemy cast bars
48 can mount in some areas, where before you couldn't
49 flight paths connect to each other, even if they are as far away from each other as can be, in classic you had to get closeby ones before connecting to the further away ones obviously
50 auto loot without having to press some other key in addition to clicking the mob
51 mounting up is faster
52 rare mounts do their noise too, when you press "jump" while standing instead of only epic ones doing it
53 can use quest related items straight from the onscreen quest tracker UI, instead of looking for them in your bags
54 the aggro percentages are shown
55 people's movement in bgs is shown fluidly on the map, instead of their positions moving every few secs or so
56 quest items have a sparkling glow to them
57 professions give you perks in wotlk
58 you can right click on items in your bag and they will be put into your bank when you're there and vice versa, instead of dragging them there
59 when you put an item into the auction house that is the exact same as the previous one, then the AH adds the previous price you wrote automatically, instead of you having to add it manually again
60 you can shift click items from your bags when the AH window is open and it will copy/paste the name into the search box of the AH
61 you can right click items from your bags and they will be added to the AH when you're there, instead of dragging them
62 you can open things like clams while mounted
63 you can open things like clams while moving
64 there's a search bar in the profession windows, if you wanna find a certain recipe quick
