The planet defenses.

[JetJaguar2000]

I would propose that homeworlds be given special "strong" status, but that new colonies be much weaker and take much longer to reach "full strength." As the previous poster points out, it takes only a few hours for a colony to hit 400mu, at which point it is effectively indestructible until much later in the game. This allows for lots of annoying plays by land grabbers (I still don't understand why land grabbing isn't kept in check the way attacking homeworlds is, but ok) who can take ownership of huge swathes of the game very quickly.

Building a colony should be a very risky proposition, especially if you're considering doing it in an area where it is likely to be attacked. Right now, you can swoop in to an opponent's space, knock out an outpost with a very small fleet of ships, and drop a colony ship in its place in a matter of minutes. If you can avoid a counter attack within, literally, 2 hours, you're golden. You now have an effectively permanent foothold in their space, and there is nothing they can do about it.

EDIT: Btw, I think you guys should really reconsider how you frame the strength of colonies. I don't know if this bothers anyone else, but the idea that an "undefended" colony is actually capable of handily obliterating everything up to huge fleets of capital ships just feels bizarre to me. It makes you wonder what role the explicit orbital defenses are supposed to have. A laser defense array, as strong as it is, adds literally nothing to the strength of a colony. It's peanuts in comparison. I think colonies should automatically build some kind of defense units that are observable as orbitals, if for no other reason than to give some visual parity with the actual defensive units.