The lack of a single player campaign is starting to make me less and less interested in this game. I don't see why they couldn't have included that (note: I'm not a developer, so I don't know how much money or effort is required to throw in bots or whatever). I'd have paid full retail for a single player offering with a fun story featuring Crazy Dave. Instead of "hells yeah, preordered!" this one is wait and see.
The reason (as it was explained to me) is that they're testing the waters with this one to see if it does well. They're putting everything into the multiplayer features since they believe (probably correctly) that more players prioritize multiplayer in shooters over single-player. They have limited resources to work with because the game is an experiment (crossing over a casual brand into a more hardcore genre) and also, time is limited because they want this one to come out as close to the Xbox One's launch as possible.
If Garden Warfare sells well, the sequel will very likely have a campaign. Gamers just need to prove to EA that this is the kind of game they want to play.
Also, they told me you can play the offline co-op mode by yourself on Xbox One. Probably not on 360, but I'm not sure.