I think the fact that co-op is what puts the game in its best form is what gets me the most.
An enjoyable experience is an enjoyable experience, and I look forward to playing co-op with my brother, since we had fun with the demo. That said, it just kind of sucks how Resident Evil 4 was INCREDIBLE as a single-player game, and RE5 is only adequate in comparison.
The idea of Resident Evil needing to be better with a human partner is just bothersome, in a way. I really miss that feeling I had in REmake when I'd be playing alone, at night, with the lights turned off, and it was so creepy and tense, while fun and awesome. Such a memorable experience, and very much differentiated from typical gaming fare. Resident Evil 5's focus on action, especially when the market is flooded with pure-action games today, doesn't help.
I'll still enjoy the game, of course. I already had a blast with the demo. You can say "that's what matters" and I won't disagree. I'm just noting the potential downsides of RE5's direction.