The original Doom and Wolfenstein 3D weren't designed for modding either, and look how many TCs there are for them (in case you don't know, tonnes)...
I agree with you about how hard it is to create entirely new areas, but a TC doesn't have to use brand new ones. There are, after all, plenty of areas from BG1, BG2, IWD1, IWD2 and Planescape to make use of... plus you can easily modify them in IETME or even Paintshop Pro or Photoshop;). From what I've heard of upcoming games like Doom 3, future FPS editors are going to be even more complex and laborous than 3Dsmax, plus it sounds like it'll take something like 10 minutes just to make a single texture, what with all the extra layers you need to apply (bump mapping, etc).
Creating a new creature also takes a heck of a long time and effort in the IE, but the same is true of any sprite-based game. I cite Doom again as an example - it has TCs of Aliens, Batman, Ghostbusters, James Bond, an anti-terrorists TC, a western TC, and numerous others. Starcraft is another sprite based game that has tonnes of TCs (admittedly area design for it is a cinch).
Dont forget warcraft 3, the cutscene making in that game is brilliant, you can make a custom map that is only a cutscene. Usually these cutscenes involve two 2-headed ogres fighting over a remote, and as they fight, they switch channels, which is funny. Let me know if you own warcraft 3, the frozen throne.
On another topic thats on-topic, I think that ie games are one of the few games out there truly worth your effort in modding. They proved great foundations for ideas and stuff, while not stamping out your creativity with insane graphics and such...
I for one cant wait till Tower of Iron Will, my Tc doesnt look all that promising at the moment... I think Chaz58 left the team, so we are stunned for the moment. However, ideas are still flowing, so we should soon be able to get back on track. Also, any work i am doing with modding is on hiatus until i manage to get my copy of bg2 back from my "friend." I cant believe i borrowed planescape torment and lent out bg 2, torment sucks!