Anti-creative-mode. It's like rolling up a 100th level D&D character - okay, so you can do everything, that's fun for about an hour.
I would like to play in a server with everyone doing a community thing. It would be pretty cool to do a futuristic city thing with sewers and stuff. My problem with Buildcraft etc. is that the automation of resource gathering and refining is just stupid and boring, and what really matters is what cool new things it lets happen in the game.
Wants:
Conservation of water / lava (irrigation, rain gathering) DF style
Melt of snow and ice if temp is too high nearby, freezing of same if temp too low
--These two give us another resource which needs to be moved around and used rather than just dropped with a bucket and forgotten.
Absentee shops - stealing would be nice but it's super complex to deal with criminal stuff and guards etc.
-- This gives us a reason to get together in a town besides just looking at each others' buildings.
More redstone output devices like pistons, dispensers, and doors
Such as: winch (raise / lower the block its rope is attached to, keeping any rails or whatever that are on it. Also a piston should go through the rope without affecting it)
--Lets us make cool contraptions
About 5x the number of generated structures, both in variety and density
--Because exploration is pretty bland right now
Regional monsters / treasure
--Like emeralds are now only in extreme hills
I don't know if these are mods already, and obviously we can't really expect to make a DF mod pack (although I would be excited to work on that once the modding is easier with an API). It's just the kind of stuff I think is important.