Strategies are already known for all the bosses, there will be better addons than vanilla had, especially since the engine will be updated. Players overall got way better than they were in vanilla. As example a lot of players in vanilla were clickers even at top end of pve. A lot of early 40man bosses like Onyxia and some MC bosses can be undermanned with like 20 competent people. The biggest blocker might be gear since you only got 2 items per boss, or none of the top players actually playing on that server. Even with gear being a block dps checks wont feel as tough if everyone min/maxes. Back in vanilla it was common to have lets say 2 hunters and one hunter doing like 50% more dps than the other. If everyone min/maxes to even 90% of their potential the dps checks won't be as rough.
As for C'thun, he was heavily bugged originally and I assume blizzard will release bug free version.