SO I was wondering, what kind of changes make a game different enough to the last game that would make you want to buy it?
I think that FPS normally change enough through games to make them worth buying, but games like Fifa probably don't change enough to make it worth buying every year, or maybe not even every 2 years...
Halo reach has new weapons, abilities and armor. Waaay different from any other Halo game in my opinion.
For me it's mainly the storyline that keeps me captivated. If a game has a good storyline like Halo has, then I will buy the sequel. But also new features and items etc. need to be added as well.
It's a good question. I think it's a culmination of things, from playable demos to promotional work done by developers and distributors. If you have a franchise, people will be hooked onto and will never defect from it no matter how terrible the games, I would use myself and the NFS series as a prime example (ProStreet FTL). So this doesn't push the developers to change much, because they know they have the fall-back of the franchise faithful that will generate a fair bit of revenue on its own. Remember: they're all out for money - very few are community-based.
The biggest gaps I've seen from FPS sequels are from Halo 1 to Halo 2, and Call of Duty Modern Warfare to Modern Warfare 2. I think the developers don't change much for a) the aforementioned reason and b) because they want to stay true to what made the game so faithful in the first place. EA are the most guilty of this: their FIFA, Tiger Woods and NFS series have all been fairly similar over the years, with just a few tweaks and new features to boot with each new instalment.