The core rotation for most specs isn’t that bad, but the number of extra/optional/situational skills is overwhelming.
For example, I can get away with a 4-5 button rotation on my BM hunter, but I can’t fit every ability in my spellbook on 2 action bars. I spend a lot of time opening my spell book to use non-combat skills/hearthstones/etc because I hate having more than 2 action bars on the screen.
I think 8 buttons that include all of your combat abilities (damage,cc,healing,buffs, and other cool downs) would be the sweet spot for me.