I don't really know what exactly happens when you blow a planet up. Only thin I know for sure is that there's no ground assault. And no ground to place your Hyperthingy. Also I think it would be good to know if you can still build the big ships on planets you blew up.
It may not be smart to blow up MC, because of your ISDs. Suppose you don't have an ISD building planet and MC is quite easy to take over. If blowing the planet up means you can't constuct ISDs there, it would not be a good idea to blow it up, since in that case they have MC cruiser, and you DON'T have ISDs. I also can image that due to their importance some planets can't be blown up. This could also apply to MC Kuat and the like.
The thing Teradyn mentioned about what happens to the bonus is also quite important to know.
Orao also has some good ideas with the fighter launching. I've had tons of times where my bombers are launched when I'm attacking a space station. Not a bad thing, but they didn't launch anywhere near the station but in the beginning of the battle. Now if I were the captain of that ISD, I would have ordered the launch officer to take a walk out of the airlock, and with no spacesuit off course. Or perhaps some 'apology accepted lanch officer Needa'. Lucky for me Boba usually messes up the enemy fighters and bombers. And I can order my TIE Fighters around a bit so that they appear nicely behind the enemy fighters and bombers, really effective.
And I'd also like to know whether you can block trade routes by placing a fleet in orbit of a rebel planet. That would make for some economical blockades. Perhaps a smugler can restore trade when you use him on a blocked planet.
Also the number of buildslots on a planet is quite important. You can always turn them into a large mining facility. Really nice if you have that planet that doubles the income from all mining. And some front planets should have large garrisons. Though I always buy only one of each troop producing building.