I've found that writing a science fiction story is very different from writing a fantasy. This probably sounds absurdly obvious, but I think you need to actually try writing both before it slams home just how significant the differences are.
For me one of the major issues is voice. I have to use a completely different voice for a story set in the 2100's than I do for a fantasy in a medieval setting. I actually find that fun, and the few readers I have had so far seem to like this voice better than my fantasy voice.
The hardest part for me has been dealing with technological issues. For almost any action that a character takes, I have to ask myself whether the means of performing that action would have changed over the years. It's not just a matter of whether a character will have a flying vehicle or not. Will you need cash in any form? Will you need to even use thumb or eye scanners, or will advances allow, say, your apartment door to recognize you in some even more advanced manner, such that you don't need to actively do anything for it to open up and let you in?
Since basic mind/data interfaces are already being developed, I expect very advanced ones to eventually become affordable to the masses. Imagine inserting cards into your head to directly access data, from languages to history to...whatever. In that case, I think the need for judges and juries would evaporate, since the authorities could simply plug into your data interface and scan your mind directly to see if you are guilty or innocent. You may no longer have to take tests, as they could simply scan your mind to see if you have the requisite knowledge and understanding. My story begins with a character graduating from college after plugging in for about twelve minutes for the university to scan him. I have to think like this for everything, even the smallest daily activities. This really slows things down, but if I don't do it I will end up with a story filled with inconsistencies.