I'd add a few addons here.
-BugSack
-!BugGrabber -Together these two record the UI errors and instead of getting that popup screen, it just says "fatality" with a cute voice when an element crashes. Very useful when addon decides to go into an eternal crash loop during a boss fight. (Although you get tired of the voice very quickly at that pint)
-BadBoy: Spam Blocker - Removes advertisement spam from trade chat and gives an easy click option to report the spammer. Quality of gameplay improvement.
-WeakAuras 2 - A lightweight custom graphic addon. Can display basically anything on virtually infinite amount of trigger options. Excellent for tracking internal cooldown of trinkets, creating graphical display of buffs, etc. Best of all, the auras can be shared through "strings" and are found in abundances online. Raid wise, creating boss timers, or massive cooldown icons on your screen for bad debuffs, can be a great help.