Have you played the intermediate tutorial? It is a full campaign with no time limit. It gives you a good feel of the gameplay
I disagree. I think this game really shines when you are playing with a few other people...solo it's much less appealing.
This is definitely a game that many (myself included) find most fun in co-op. But if you're looking to find out about what the game is like, etc, then the intermediate tutorial will give anyone a good idea if that's something they'd enjoy playing more of, in co-op or not. I've had a few players telling me that I give too much away with the demo, and that people might not buy it if they get that feeling of satisfaction/completion from the demo (which I think is bogus), and I've had a few, like yourself, who have felt like it is on the shorter side of things.
Generally speaking most players seem to buy it, if they are going to, by the time they are finished with the intermediate tutorial or even before. That's been working pretty well for 5 months now, and has been praised by a number of reviewers as being unusually meaty, so I think it's tuned about right. I agree that you can't get a full feel for the co-op aspects in just an hour of the campaign, but that's not really the point -- you have plenty of time in that scenario to explore the co-op mechanics to see if they are to your liking (and you can start as many 1-hour campaigns as you want, for that matte, if you want to experiment with different starting scenarios), but the main selling point for a game is always going to be "is this fun to play?"
My suggestion is generally to just send your friends the link to the demo, and get them to go through the tutorials. If they think it's cool, likely they'll be sold on it by the time the intermediate tutorial concludes, anyway. Or at least they'll know whether or not this is the sort of thing they'd be likely to enjoy in co-op. Hope that helps!