There's a built in colony limit in the game -- the number of planets. Want fewer colonies? Play with Rare Planets. Your complaint would apply better to a game like Civilization where you can build cities on every little square.

Agreed. Since you can always reduce the amount of habitable planets before starting the game, you can stop the colony rush that way.

What about a gradually increasing limit? The number allowed would go up when researching certain technologies, swich to better governments, ext., the the "rush" is more spread out.

If there are enough planets then the rush will stop before they are all taken. The 12 maintanence per colony can really add up and eventually you just have to wait for you population to grow to increase your tax base before you can afford to colonize more.

If you want to take out the rush, then reduce everyone's starting money. This cash balloon at the beginning is what allows the rush. Make the starting money zero and see what happens.

If thats not enough you could also decrease base population growth.

Set rare planets, rare habitable, rare stars, and a gigantic map, slow tech, no tech trading, and you'll eliminate the colony rush. Finding habitable planets will be like panning for gold. With a full board of opponents, the average will be like 6 planets each (on a gigantic map!). Of course a human player will have a huge advantage in such a setup because the AIs won't know to up their research. Maybe you can limit yourself to one lab per planet until all the planets are filled, to square things up.

limithing the habitable worlds does not eliminate the colony rush, indeed, the race becomes even worser, because you realy realy need as many planets as possible.

as for the conquered worlds: I said the further away a colony is from the capitol, the harder is it to keep up the morale. Conquered worlds would NOT count towards the limit, but the morale would be even worser. (btw I never researched space invasion before republic, or 2nd logistick update)

btw why is GC2 not differing bettween the "colonists" races like in the most other 4x games. they couldh have some impact on the colony. ie. drengin colonists increase the military production of a world, iconians the research... this way we would end up with different setings for planets.

it was an example noone says it have to be 10. but it can, imperial 10, +5 / logistic level, +10 for new government... this way we could end up with 65 colonies, thats quite a good number. Occupied worlds still dont count towards this number, so we cna have 200, but with low morale.

It is not just about me, it is about the AI as well. If the Ai wouldn rush for colonies, I wouldnt either.

Im on a mod, will release the beta on the library soon, with a slow start. Cargo hulls and colony pods are very expensive and have reduced capacity, but during testing, Ive seen no difference, the Ai still rushes for colonies, it is slower, but still a darn rush.

A limit is the only way (that I see). And as said, this can be made an starting option, so no need for those of you, who dislike the idea.

Having had a little time to think about this I agree with the sentiment of the OP (but not the solution).
I think there should be real problems associated with having a big empire. Morale is the obvious one to suffer first. However you could also have problems with economy and production based on "corruption" or higher colony and maintenance costs based on distance from the homeworld.
All large empires thoughout history have fallen. In the real world large empires don't keep getting stronger they actually reach a point where they lose cohesion and start to fragment (maybe a new civ based on your own tech/planets, taking away some of your navy too). This may turn the play balance towards long term sustainability rather than expand ASAP at all costs, which seems to be the way at present.
I'd actually be happy for there to be no real solution to the the "big is difficult" problem. Just some things to minimize some of the worse effects such as cetain government types.
The biggest problem this would overcome for me though isn't the initial rush, it is the opportunity to address the "after the hump, the game gets ridiculously easy" problem. You could then have games which see-sawed rather than the present inexorable procession of your civ catching up and then crushing the others. (Assuming you lived long enough in the first place).