2.) trees cause cookie-cutter builds and that is one of the prime reasons it was removed. After 8yrs it proved to be a flawed MMORPG design, especially for a game with a community so nerdy they came up with theorycrafting.
3.) The only thing I will agree on is that 10man needs to be buffed; its far too easy and way to attractive now-a-days (look at how 25man's are an endangered species now). But flex is good.... it needs to stay.
4.) double-edged sword here, and WoW has a whole shit storm full of them. Become too distinct (vanilla days) and you become too specialized for the encounter/content to the point you're either desired or shunned(I am sure every holy paladin who's guild did sunwell will agree). Become too generalized and you fix that issue but then lose the specialized needs important for an encounter.
6.) no. it reduces the stress of needing 10,000 of one currency for all upgrades when you could only need 2,500 badges, 4000 coins, and 1200 of another. Before you say "whats the difference?" understand that the rate you get each currency is different than just a single one. So you could get 2-3 upgrades in one day with multiple coins rather than spend 3 days with just one.