Firelands was small and lasted for quite a while, and the massive gap in skill and gear required between the first 6 bosses and the last boss on Heroic, was kinda silly.
The expansion itself was a massive World of Portalcraft one: nothing felt connected, everything had loadscreens or queues.
Dragon Soul's tier was shitty in the sense that you had to take a portal or flightpath to EVERY bossfight, further contributing to World of Portalcraft. The entire instance was stuck in a square cut in a wall of the caverns of time, with some art assets thrown in that hole with some instance portals. This is by far not as nice as Black Temple, Sunwell's isle, Icecrown Citadel, or Ulduar.
The inside of the instance was 99% rehash: Wyrmrest temple. Maws that were used in Twilight Highlands. Eye of Eternity. More Wyrmrest temple. Gunship. Only Spine and Madness were somewhat new in environment, but grossly compensated negatively by starting with a loadscreen and a ton of RP, and shitty mechanics. I was warlock, Spine HC. Yeah. Final fight consisted of peeling off plates of the back of the boss, and scratching loose his nails and chin so that the NPCs could do the real work. Oh, and Thrall turned from a badass warchief into a massive sissy, the whole Aggra thing as well as the Dragon Soul beamshooting thing was just cringeworthy.
The DS loot system was the worst ever: For the first time ever only dropping 1 normal item from tokenbosses, and no coins or whatever to compensate. BiS trinkets stuck behind an RNG wall, only 6 bosses had a miniscule chance of dropping one of these, as well as the only item for some classes stuck on those bosses that only drop 1 out of their 12 normal items (IE: tanking shield, took us 9 months). I personally ran all bosses in DS from release to Cataclysm lauch: some items just only dropped once or never on 10man.
The DS nerf system was the worst ever: Blindly after 2-3 months, EVERY MONTH WITHOUT FAIL, 5% nerf. As someone who finished Madness HC with 15% nerf, we just did not get a chance to properly progress on some fights. You looked forward to Ultraxion's DPS check? Let's get 5% nerf as soon as you get there. Not soaking on gunship consistent enough, truly having to progress? 10% nerf in your face, and it doesn't hurt anymore. Trying to min-max on Spine? Let's throw a 15% nerf in your face so you can just do it without having to try. From 20% and onwards: Abilities that should oneshot you were survivable, making farming DS a complete borefest. These nerfs shouldn't have affected Heroic, period. The Spine Tendons HP nerf was a good one, flat nerfs had to be replaced with targeted nerfs.
TLDR: 90% that was bad was DS, and the fact that DS covered 55% of the duration of the expansion, made the overall expansion bad too, especially because the previous 45% wasn't that good that we could forgive it that easily.