/b/ has always sucked.
Now that's out of the way, unless you're referring to some current event I'm unaware of, the problem is the rapidly-expanding userbase. Previously (in the early days, when it was still fun) the culture was "lurk more forever". Lurking for at least a month before posting was the etiquette, and failing to do so would get you shouted down universally as soon as you demonstrated any ignorance of the still-complex but then-manageable memeplex.
Nowadays that culture no longer exists, the newbies outnumber the oldbies in any given thread, and the new users reenforce bad behaviour in each other. The reason for this influx of new users is primarily due to the media coverage of the scientology project, though the trend started before that and breaking into the mainstream was always inevitable.

That's all common knowledge. In my personal opinion, another major contributor to the suckiness is the current internet-wide notion that "trolling" simply means "saying something dumb" as opposed to "setting clever traps for idiots". So now that the definition has been lowered enough for any moron to do it, the morons do it relentlessly. Not that /b/ didn't always have crapflooding, but now it's done with attitude. This example and other prevalent behaviours mean there's little reason for older users to stick around and contribute the way we used to.