Perhaps the greatest bottleneck for humanity exploring the Solar System in person or even with swarms of highly sophisticated robots is the lack of the means to get from one celestial body to another.

Historically, getting into space and sending heavy payloads across or out of the Solar System has relied on chemical rockets. They do the job, and very well, but they suffer from the fact that even when the first German V2 flew into space in 1944, these rockets were already operating at near their theoretical limits. True, a lot of progress has been made since then, but it's mainly been in the realm of tweaking the system and cutting down on weight.