First, OP, why didn't you like the Flood? They change up the gameplay in a way that can be really interesting. The people I know personally who have troubles with the Flood have them because they can't adapt to fighting them. They keep thinking that Halo "plays the same," when it really doesn't.
Then how the fuck do you fight the Flood?! I still think they're bullshit and quite Anniversary at the Library. I just don't like them in terms of lore or gameplay.
With that out of the way, there are two really big reasons why you should pick up Halo 4:
1. Halo has absolutely excellent gameplay and mechanics. There are so many interesting ways to approach any given combat encounter that it's a blast to mess around with.
2. They've got a rockstar team. There are ex-Rockstar guys, ex-Retro guys, ex-id guys, ex-Bungie guys, ex-Naughty Dog guys... It's a huge cast of some of gaming's best and brightest.
Also: they're releasing a ton of free co-op content, so there's that.
But will it do this in a way to live up to CE?
*sigh* I guess there's some space in my GameFly schedule...