Not sure if you're asking in general or specific ideas...
For me to pay money for a game, it needs to pick a niche and focus on it. Halo Reach for example has really polished online multiplayer. Bioshock has an awesome story/characters. Ninja Gaiden 2 has great action. It's better to be really good at one thing than mediocre at many.
I also look for good replay value for any game that I buy.