UI

This might be a little long, but hopefully not too bad. Probably end up with one post about UI principles and categories of mods, and then one post that shows my UI and why I do things the way I do.

In most custom UIs, there is not actually much difference in the information present than with a Blizzard setup. The primary differences lie in where that information is located and how it is presented to the user. The one thing that must be considered in making your own UI is whether or not you can get the information you need quickly when you need it. If you are running high on threat, will one of your mods tell you or can you see this in your peripheral vision? If you need to check your health, can you do that while monitoring your GCD? Can you tell how much longer it will be until your cooldowns are ready again? If not, maybe something needs shifting. Many mods are very customizable, allowing you to do almost whatever you want. There are quite a few types of mods that are included in many custom setups, as well as many more obscure ones.

Pretty Common

Unit Frames - Pitbull, XPerl, ShadowedUF - These are the frames for player health/power, target health/power, etc. Customizable emphasis and positioning in particular are key.
Raid Frames - As above, plus Grid, Vuhdo - Large improvement over Blizzard's standard raid unit displays.
Bar Mod - Bartender, Dominos - Allows you to move, resize, and otherwise distort Blizzard's action bars. Also allows easier assigning of hotkeys in many cases.
Boss Mod - DeadlyBossMods, BigWigs - More or less required for raiding, gives alerts and timers on all kinds of boss abilities.
Chat Mod - Chatter, Prat - Allows the adjustment of various features, generally fairly minor.
Combat Text - MikScrollingBattleText - Allows notification of incoming and outgoing damage and heals, gaining and losing buffs/debuffs, target gaining buffs, etc.
Threat Meter - Omen, Skada - Allows monitoring of threat in a more customizable way than the default.
Damage Meter - Recount, Skada - Allows monitoring of damage done, damage taken, healing done, etc. without reading your entire combat log.
Cast Bar - Quartz - Repositioning, resizing, custom colors, whatever.

Less Common (pretty much arbitrary, sue me)

Death Reports - Acheron, Recount - What happened before someone died.
Font Changing - Fontain - Changing fonts for aesthetic or functional reasons.
Minimap - SexyMap - Different shapes/sizes/looks for the Minimap.
HUD - ArcHUD, MetaHUD - A lot like unit frames, different presentation.
Nameplate Mod - TidyPlates, Aloft - Customization of the nameplates that can be turned on.
Proc Notification - PowerAurasClassic - Not sure how to categorize this, basically lets you show an icon based on any of a large number of criteria.
Buffs - SatrinaBuffFrame, ElkanosBuffBars - Allows presentation of buffs in different location/way.

There are all kinds of other things out there, including many I use that I had no idea how to categorize. Anything I use that is visible in my UI I'll talk about in that post, plus I might make a post on some of the more interesting/customizable addons out there.