Even if you're not lagging or DCing yourself, you could be causing issues for others if you don't update or disable some functions!
(Summarizing this post in the WoW Forums who's source was this ElitistJerks post.)
1) Remove Recount-
Disable Recount Syncing (/recount synch) or disable Recount
(Ami: This was my issue, I switched to Skada and practically all my 25-man lag disappeared.)
2) Disable Blizzard Combat Log-
Right-click your combat log tab > "Self" > "What happened to me" > "Everything" > Click on each of those, and then click through each of the sub-tabs and disable EVERYTHING. Make it so that NOTHING appears on your combat log.
(Ami: If you run a combat log record for uploading to World of Logs, this will make it not work, so either have someone else do it or don't do this step, however, this is reported to be one of the biggest fixes, so up to you.)
3) Find out what addons from others are spamming you-
This amazing addon allows you to monitor addon message traffic from your raid members. What you want to do is to MINIMIZE this traffic. Many addons have secret "whisper" programs that we cannot see. They whisper other players in the raid/party for all sorts of information. The problem is, many of these mods do the whispering extremely inefficiently, thus leading to massive, MASSIVE spamming of your client with data.
Main one to look for is Gearscore. However, the author says he's improved the way it shares info and to make sure everyone updates to fix it. Several people mention banning Gearscore from being on during guild raids.
(Be careful: some addons like LibHealComm(X) have to do it, other addons like X-Perl and Carbonite and GearScore don't. Carbonite also doesn't have to- settings, under Privacy, and then turn off all the broadcast information boxes and channel options.)
4) Latency Improvment-
Lowers your latency by changing how your computer handles information acknowledgments. It's amazing. This is NOT an addon, it is a modification to your computer itself.
Install it, try it for yourself over a period of a few days. If you aren't happy, remove it. Make up your own mind. It won't break anything. Most people report a noticeable improvement in button response times, as well as smoother mob movement patterns.
(Ami: I went from ~150ms to ~30ms. Oceanic or overseas players see more significant improvements as shown in the comments section.)
I know some of you already know all of this, but I hope this helps anyone else who might be having issues. I myself blamed it completely on Recount until I stumbled across this earlier. Now I know that it was a combination of Recount, recording the combat log, and the raid leader and others having an old-spammy Gearscore.