The game is woefully short. I've only played maybe 5 hours and I'm almost on mission 10 of 11. I've also completed all 5 bonus missions, each take about 15 minutes.
There's actually at least 6 bonus missions; look around on the net and I'm sure you'll find the one you're missing. I unlocked them all on my brother's PS3 when I was visiting him this past weekend.
I must be a horrible player because I've already put 10 hours into the game and I'm just now starting mission 6; mission 2 took me 2 or 3 tries, and mission 5 took me a while, too, because I got myself into a bad situation trying to extract the helicopter crew without having cleared enough of the opfor out before the checkpoint...had to redo the mission and do things differently. I agree that the total content does seem lacking (I realize I'm just slow at getting through it), but I'm willing to hear what DLC they're offering, and will probably pick it up if it's at a fair price.
I bought Arma back not too long after it came out (before it came to retail in the US), but I couldn't get into it and the performance was horrible. A couple of years later after I got a new machine I tried to revisit it, and found I couldn't get it to run in Vista unless I reduced the amount of RAM in my system (it had issues with 4gb 32-bit systems), and I just found that totally unacceptable. Apparently it had been an issue for months and there was no discussion from Bohemia that it would be fixed any time soon. I avoided the freight train wreck that was Arma2 as a result, and from what I've read I'm pretty happy with that decision. I think a more "consolized" milsim is up my alley anyway; plus it'll give me something to play with my brother who only games on consoles. I picked up a PC version for the singleplayer experience and for use of the mission editor (and because I prefer KB+mouse), and I'll probably pick up a PS3 copy sometime for the multiplayer component to play with my brother.