When is anyone at bitcointalk not BULLISH about bitcoin? It's not like they have a financial incentive to let it tank.
Some people are there because, if you can believe it, they are actually intellectually interested in the complex and novel dynamics (sociological, economic, technical, etc) found in bitcoin (or that may potentially develop as the system grows).
It just happens to be a community that has had a great interest in creating and seeing bitcoin succeed. Of course, with growing numbers, the noise hides the signal, but take a look at the relevant subforums and you will indeed see serious discussions about bitcoin viability, previous challenges, future challenges, network security, scalability, etc.