Ostensibly, they start along the lines of "my firm is implementing new systems, what software should we choose..." Then, very quickly they become a mud-flinging game of mirrors, misdirected passion and other more general angst (Why does Program X hate me?) They are littered with sarcasm, attempted yet misunderstood humour, and a few genuine users who take the time to relate their personal experience.
A great example is this ArchiCAD vs Revit discussion. Please note that it is in the ArchiCAD group (meaning your submissions may be moderated by a pro-ArchiCAD discussion manager).
Another similar discussion: