I'm sorry, but I don't have anything "intelligent" to say today (I know, I still need to do "Why do I WoW" post - I'm working on it but it's not ready yet), so I'll take the easy way out and post my UI and addons. Please remember this UI is
not for raiding, and while it's usable in BGs, I don't think it's particularly good for it. It's mostly designed for
solo leveling, and to be fairly consistent between different characters (I don't want to start hunting around for my HS every time I log into a new alt).
So, without further ado, here is Enara's UI. I'll try to walk through it in some sort of order, so no one gets lost along the way.

So first, we can see
FuBar on top. One bar is enough for me, and I found having the bottom bar annoying when trying to click stuff on the action bar (I'd move the mouse down too far, and click FuBar instead of whatever it was I was going to cast).
On it we have
LocationFu (so I don't get lost) and
MonkeyQuestHelper (works with
QuestHelper and
MonkeyQuest, which is nice) on the left side. On the right side, we have
RestFu (keeps track of all my alts' rested XP),
MoneyFu (keeps track of gold on all alts in that realm), and finally
Baggins.
Under FuBar, you can see my combat log - I don't use that part of the screen for anything else, and I don't usually have to click there, so it's ideal. The chat log is still in the bottom left, in its default location.
Next, around the mini-map I have icons for
WOWecon (that I talked about a little
here),
ACE preferences,
ItemRack and
Gatherer*. Just beneath it is QuestHelper - while it's very useful, it might be
too useful and I'm still a little undecided on whether or not to use it**.
For my inventory, I use Baggins, and it's excellent. I had to disable all my addons temporarily a little while ago, and the default bag management was just horrible.
Finally, the 2 teleport spells on the left (and, if you can see,
Arcane Intellect behind Baggins) are brought to you courtesy of
ClassButtons. While it isn't perfect, and often misses a lot of spells I would have liked to see for some classes, is still very useful. Who knows, maybe I'll finally get around to adding the spells I want to it some day...
I also use
Auctioneer for the AH,
Cartographer and
TomTom*** for my map addons, AckisRecipeList for trade skills, Linkerator for linking in chats, and Omen when I go instancing (or soloing with a pet).
Finally, the default Blizzard UI works for me, with the spells I use in combat on the main action bar, the more situational (and less used) spells in the bottom left (and bottom right, if there are too many of them) bar, and the food/drinks/HS/potions/trade skills on the right action bar.
Other than that, I have some class-specific addons - Hunter's Helper (for all your pet management needs) and Smarty Cat Mana Bar (to keep track of mana while shape-shifted on Druids).
And, of course, this post wouldn't be complete without mentioning
WowMatrix, which allows me to easily keep all these addons updated and making patch days relatively pain-free.
UPDATE:Stupid, stupid, stupid... Forgot one of the most important addons for us altaholics -
SendSelf. A fairly simple addon, all it does is add the names of your alts to the auto-complete list when sending in-game mail. No more sending mail to random similar-named people and no need to add them all to the friends list. Must-have for anyone with more than 2 active chars on the same realm.
* I've heard Cartographer can do everything Gatherer can, and I tried, but it didn't really co-operate. If anyone has any suggestions on getting it to work, I'd be more than happy to use one less addon.
** Without QuestHelper, you actually have to read quests to know where you need to go. With it, I often find myself just running towards the arrow to get the quest done.
*** I generally find the arrow/pointer thingy too annoying. The minimap indicators, however, are pure gold.