well the new units in Brood War brought a significant change - they weren't "gimmicks", but produced new, viable strategies. I don't really have to tell you what a group of valkyries can do to a horde of mutalisks clumped up in one spot, right?

or how devastating can be an early deployment of two lurkers between the opponent's crystal field and his HQ

dark archons and mind control, terran medics and their impact on infantry warfare, dark templars, it all has a decent place in the greater scheme of things. the balance is shifted, and players have to adapt.
in morrowind and oblivion, the new creatures just do the same stuff but just add a new 3d model. in the end they're either hacking at you, throwing fire/lightning/frost at you, or debuffing you with damage health or whatever else, but it's all still the same stuff that the original game had. whatever you were doing with your character in the base game, in Shivering Isles, Bloodmoon and Tribunal, you still do the exactly same thing.
as for class levels in TES, true enough, the system is different. but what they could do is add new types of weapons, new side skills, maybe some sort of a crafting skill? new school of magic, whatever, just so that it expands the ways in which you can develop your character. as it is, playing Shivering Isles is no different than playing base Oblivion. that's why I think it's not the kind of expansion I expect.
true enough that the modding community is an integral part of TES games, since the engine and game release that includes the construction set strongly support player creativity, the same way that WoW's liberal interface policy spawned a throng of various interface mods, ranging from custom unit frames and action bars, through raid warnings and class-specific addons to stuff like organising whispers into an instant messenger window system and in-game bejeweled. personally, right now I have exactly ZERO elements of the original UI, everything's customized. that's all cool. but it's seperate from what the developer does. if you compare the amount of content at the release of the game and when the first expansion came out, it's like 2-3 expansion packs of the Morrowind quality. not to mention that there's always the ongoing effort to balance various classes in a great deal of aspects (solo pve, group and raid pve, battleground pvp, 2v2 arenas, 3v3 arenas, 5v5 arenas, flexibility, group buffs, there's a tonne of interactions between classes and really balancing that shit with a bunch of whiners constantly crying on the official forums is a titanic uphill battle)
as a side note, a curious thing with both oblivion and morrowind... even though Morrowind is a quite old game, the quality of scenery graphics is still "viable" in 2009, and will be in the future - just like Oblivion... great job on that... BUT. characters and monsters, animation... horrid.
It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood, who strives valiantly; who errs and comes short again and again; because there is not effort without error and shortcomings; but who does actually strive to do the deed; who knows the great enthusiasm, the great devotion, who spends himself in a worthy cause, who at the best knows in the end the triumph of high achievement and who at the worst, if he fails, at least he fails while daring greatly. So that his place shall never be with those cold and timid souls who know neither victory nor defeat.