Wrath heroics and even Naxx was able to be done with previous expansion gear. That is where the line should be drawn for unacceptable ease.
Not with the crappy Karazhan gear that I had
Naxx was trivial but Ulduar was pretty challenging IMO. It all came down to how good the other players around you were. I got involved in a "hard core" rading guilds in WotLK and it just took way too much of my time, left me stressed, and left me feeling like WoW was a second job. It was fun, but not rewarding enough for the amount of time.
Then switch to "club" guilds and everything is virtually impossible because even though the vast majority are good players, there are a handful of "people's friends" who want to play and are just terrible.
The last couple of times I jumped into WoW for a month, about all that I did was PvP because it was the funnest thing that I could advance in by myself.