Personally, I thought Black Temple was pretty hard, the tuning was pretty steep even if the mechanics were generally very easy. It was harder than current normal modes, which typically a lot of people have had trouble with (though less so in SOO I suppose) and the harder fights were comparable to some of the later hardmodes.
It wasn't tuned super hard of course (as sunwell was), but there were a lot of situations where a small mistake from one player would result in a wipe (ROS interupts, lol). And the fact that so many guilds were pushing 25man progress and getting stuck on Gorefiend means that for the general population he was hard. I remember my first real raiding guild and some of the guys just could not do the ghosts, and that resulted in wiping all day... and that was a common thing for a shitload of guilds.
I later joined a much higher ranked server first competitive guild and we cleared the whole of BT without wipes like it was nothing. Sure bosses are easy when you play with the best players, but those reprisent the vast minority, not a majority opinion. A lot of guilds hit a brick wall at Teron Gorefiend, even guilds that managed to beat Vashj/KT... which is completely crazy to me, since they were multiple times more difficult.
Remember the Teron Gorefiend online browser minigame?
My guild had people practicing it before we started in work/school.
I don't think BT was that difficult personally, most of the first half of the instance was faceroll, Bloodboil could be hard, RoS was by far the hardest, and council and Illidan weren't really large cockblocks.
Tbh I think Vashj, Kael and Archimonde were the hardest encounters in TBC (not including sunwell). My guild certainly spent way longer on them than any other