Worldbuilding is easy to lose a lot of time in. Orion's Arm is a good one I've dumped lots of hours into reading. I like hard sci-fi because it makes me think they are on track for the real future. Megascale space habitats are a fascination of mine, have been since I read ringworld. Regular space colonies are also fascinating -- planets are such a waste of material when you think about it.
Utility Fog is another one of the really interesting ideas that I've spent a lot of time thinking about. Essentially it is a set of nanobots which lock arms, but can let go at intervals and thus be programmed to act as any substance -- breathable air, concrete, water, etc. It could be used to simulate gravity convincingly, or give you flight in a gravitational environment. The tiny arms would let them act as generators, capable of gathering energy from the tiniest currents of air.
I guess I am fascinated by potential practical applications. The fact that esperanto is easier to learn yet arguably just as beautiful and useful as natural languages makes me consider it an incredibly good idea. While I get the idea of lojban as a way to explore the saphir-worf hypothesis, that doesn't really do much for me. And I don't really feel learning klingon or elvish is going to do me any good in life...