Before posting an addon idea/request in this forum, please review the following list of things which addons cannot do. Use your browser's "find in page" feature to help look for specific things. Posts requesting addons that do anything listed here may be deleted or mercilessly ridiculed.
If you notice something missing from this list, or erroneously included in this list, post a comment on this thread, and I will edit the list. This thread is also posted on the WoWInterface forums, here.
Do not post here about things that violate the WoW EULA and/or TOS.
This is an official fan site, and thus must abide by Blizzard's rules. Posting about things that violate the WoW EULA and/or TOS, or this site's own rules, will get you a warning and a locked thread the first time, and a ban the second time. Such topics include, but are not limited to:
Login screen changing
Bots, hacks, or other third-party software that interferes with WoW's normal functioning
Gold selling/buying, power leveling, account sharing/selling, etc.
Addons cannot access anything outside of WoW.
You cannot read from or post to Facebook, Twitter, RSS, etc.
You cannot connect to Facebook chat, Skype, etc.
You cannot read from or write to files on the hard disk.
You cannot show what song is currently playing in Winamp.
You cannot show who's talking in Ventrilo.
Addons cannot load arbitrary URLs in the in-game browser.
The in-game browser can only display specific Battle.net account and support pages allowed by Blizzard.
Addons cannot change models, textures, spell animations, enchant glows, or any other graphics.
Anything that is part of the 3D game world is off-limits to addons.
Addons can only know about the threat tables of mobs you or someone in your party/raid are targeting.
You cannot have a multi-target threat meter.
You cannot automatically mark mobs that nobody in your group is targeting.
Addons cannot change or even see minimap tracking dots.
You cannot have an alert when a tracking dot appears.
You cannot change herbs and mines to have different icons from each other.
Addons cannot target mobs by their raid target icon.
You cannot write an addon that lets you "target whatever mob has a skull on it right now".
I don't use Twitter, in-game or otherwise, but I'm pretty sure it's in the same category as opening URLs; it's off-limits to addons. I've gotten a few reports that some types of chat frame modification can taint Twitter functionality, but it hasn't been a priority for me to investigate.