The place was tough, no doubt, but I'm surprised at your comment about bears - I ran my bear through there many times, and fondly remember being probably one of the few non-paladin tanks to survive Kael'thas' Pyroblast - we thought we had pushed hard enough to avoid it, but here it was coming, and I swapped to cat and sprinted behind a column - lo and behold, you could LoS it! And then my favorite - Vexallus, tab-target hit the orbs and soon as you know it the bear is filling the whole room and the boss dies fast. Good times.#1: Magister's Terrace - I had a terrible experience when it was relevant content playing a holy paladin, shadow priest and feral druid at the time, was never capable of getting into a pug for the heroic version because my holy paladin "couldn't aoe heal, wtb rdruid" my shadow priest couldn't CC nor could my feral druid as kitty, and bears couldn't "hold aoe threat". Even guild pugs were wary of taking me at first strictly because of the class make ups, probably one of the worst designed 5 mans in the history of this game in all categories. (Lore, level relevant encounter design, level relevant availability, level relevant time consumption and level relevant loot((I say loot simply because MT was the first of the "catchup" 5 mans that later became known as ToC, FoS, PoS, HoR, ZA, ZG, ET, WoE and HoT.)))
Bears at the time had no problem holding AoE aggro in that sort of instance - 5 mobs? May've had to tab a little, but it's not like it was Mt. Hyjal. Even tanked dragonhawk - that was tough.
Still, though, a lot of people had trouble there - once people found out you could handle the place, you couldn't log in to WoW without someone pst'ing "hey you going to run MgT?"
Back to the main question though - I can't say I can call 5 dungeons "bad"...
Old Ragefire Chasm was dull and basic. Not sure about the new one, probably been in there a couple times at most, but it seemed pretty much the same. Granted, is for minimal level people, but still - eh.
Dungeons that do stand out in my mind:
Wailing caverns - I loved the place. I hated getting lost in it. I still loved it. It certainly wasn't a bad place, but it needed better maps!
Violent Hold - probably overall the worst dungeon; I like moving through a place and doing stuff. At least Black Morass had more movement and scenery (and was reasonably tough at one time)
Grim Batol - I personally liked the place. Of course, I didn't like the fact that so many people just failed there. Still, I really appreciated how the heroics of Cataclysm felt more like BC heroics than WoLK heroics - they were damn tough in early gear, and you got a real sense of satisfaction having completed them. Mists heroics? Let's drop the pretense, they're not heroic in any sense of the word. I don't even know why they require level 90. BC level 68-70 non-heroics were considerably tougher.
Blackrock Depths - the best dungeon ever. Still is, even if it's faceroll at its intended level.
Overall I have to say Blizz does a good job designing dungeons.