Can someone recommend some UI add-ons for a returning player who's leveling now?
I can't live without:
Postal - better mailbox
Prat - better chat
TBag - better bags (shows you bank bags and is searchable across your alts)
Bartender - better action bars (super easy to set character-specific keybinds)
Clique - for middle-mouse button dispels, if that's what your class is supposed to do
ClassTimer - for better, movable buff durations and it's easy to customize
Grid - turn healing into whackamole with Clique.
Panda - one-click milling/prospecting if you need to do either.
I wish I could live without:
Some kind of unit frame replacement - I use pitbull but it's creaky and annoying to customize, but this I found is true of most unit frame addons. Most UI packages use one with pre-done customizations for solo/group/raid play for that reason alone.
TomTom - Really useful for moving around in zones without having to double check your map all the time, but addons that integrate with it (hello RareCoordinator) are extremely obnoxious about peppering your map with waypoints
Quartz - I like huge cast bars above the center of my screen so I know when and what to interrupt during a fight. But Quartz sucks.
AckisRecipe - good for finding out what is missing, but it's not so great otherwise.
Gatherer - I used to love this mod, but lately I've found it less useful and now it's just kind of there in my addons because inertia. It's more useful in a guild full of people gathering because if they have Gatherer it adds nodes to your map that you may not have ever seen.
Addons I'm not going to use in 6.0 unless it becomes absolutely necessary:
Omen - I haven't needed a threat meter since Karazhan. Threat is such a pointless mechanic.
DPS Meters of any kind - These just seem like a license to be a dick to your raid/group/etc. These days I only use them to find out how and why people died, and even then I don't really learn much other than 'oh he stood in the fire.'
oQueue - worst behaved addon I've ever installed. Ever.