5 hours? Really? Did you just speed run it on Easy without collecting anything?
I've completed the first playthrough on Expert and, whilst I agree it was shorter than I would normally prefer, I still got a good 20 hours play out of it. I'm on my second playthrough now, and I'm speeding through this time, but still collecting everything so I can upgrade all the powers and so on. I'm about 5 hours in and have just finished the north sections.
So, yes, I agree it's not the longest game, but it's far from the shortest I've played, either. And, given more content is being added all the time (the Papertrail missions, for example), I'm not disappointed with the length.
That said, the game is far from perfect, for all I'm enjoying the heck out of it.