Just Valve as far as second and third party developers goes, because there's something unique about the way they develop their IP that makes it almost impossible for another company to pick them up and maintain the Valve standard.
Half-Life - Even Valve obviously doesn't think it can deliver a game worthy of the Half-Life name, that's why there's been no third entry. Anyone buying Valve or the IP will almost certainly have to use the IP to cover such a major investment. It won't deliver, and a game that doesn't deliver is worse than no game at all.
L4D - Probably the one Valve IP which, although I really enjoy it, could be taken on and made just as well by another developer as it has been by Valve. No issues with this one.
I think, for the health of the Xbox brand in general, it is important for MS to start splashing some cash. Buying a few small-to-medium sized developers, or one large publisher, is something I'd be happy to see, so long as they were then actually let loose to make games and not just vaporware. Xbox sorely needs exclusives to remain competitive for the rest of this gen and perhaps more importantly next gen, and strong competition is the best way to keep Sony (primarily) and Nintendo (secondarily) on their toes.
I would say that almost all games become different when change dev and can't really be replicated... several times even the sequels can't replicate staying on the same dev due to the time it was developed being different, team and corporate culture, etc.
A company could buy Valve as a publisher but keep its freedom as a dev like Sony perhaps would, but I can see you having worries about what MS would do.
duduspace11 "Well, since we are estimating costs, Pokemon Red/Blue did cost Nintendo about $50m to make back in 1996"
Mr Puggsly: "Hehe, I said good profit. You said big profit. Frankly, not losing money is what I meant by good. Don't get hung up on semantics"