I can come up with alien worlds, weird creatures and, heck, even robots. But I can't come up with my own spaceships :P

I can totally see how that would be, though. People/creatures/cities are so much easier to envision, because you can at least base them on things you've actually seen in person. Spaceships...those things are intimidatingly foreign to try and wrap your mind around, especially when combined with the need to have characters say Technical Stuff. I am terrible at writing real or faked geek speak--which is one of the reasons writing for SGA and SG1 both intimidate me so much. :o