Well, for one thing, open source projects typically don't have the kinds of budgets that guys at places like Microsoft have, so I'm comfortable with the notion that they have more reason to be ecstatic about their accomplishments. Also, all of this hype is in the classic language of new product marketing, and open source projects aren't the only ones who speak that language. We hear the same "gee whiz" mantras every time MS begins the release cycle of a new OS. We hear it every time any company does something a bit different.
Just two examples of the cuff, here. Remember the marketing language surrounding the advent of Windows 3.0? "When do you want to crash, today?" Were you among those cursed with AutoCAD R13? Then R13a, 13b and, finally, a relatively stable 13c? These were just a bit longer on hype than they were on performance and they weren't open source products.
So, I have a hard time singling out the open source teams for culpability in over-hyping their stuff. Everyone who's spent any appreciable number of years using software has, at least once, run into an app or an OS that just doesn't live up to the hype of its marketing media. I'm not saying that's excusable, but there are many big names on that list, and most of those names do not belong to open source product development teams.