Scale for sure. Not just in terms of how far they were able to travel, but how big everything is and how many resources they would take to build, not to mention time.

Even if we knew exactly how to build everything in the movie, it would still take a TON of time to build it. Now start in the future and work backwards -- it's simply unrealistic.

That isn't to say that we won't have tons of things even 100 years from now that we can't imagine currently. I just tend to think the unimaginable is more easily attained than the evolution of those things already made.