I never trust a site on a .org unless it IS an organization (save for a few exceptions). I highly doubt I'm the only one. It looks unprofessional and very sloppy to have a just-anything site on a .org.
Admittedly from a newbie perspective (but then, it a perspective of a consumer!) i'm the opposite
At first sight - I think we are really talking here first sight, because once you get on the site you usually have more or less good feeling quite soon whether the site is trustworthy or not - it looks unprofessional to have a generic term coupled with .com, unless the generic term is the name of the company.
.Com still inevitably associates with commerce. But what is immediately commercial about, say, BaseballGame? The answer is, nothing. When you hear words "baseball game" you first associate them with game, entertainment, sport, competition, activity, and the matters of commerce come only second, as means to enable those primary qualities. Hence when you stumble upon baseballgame.com, subconsciously it looks wrong, because what is special about baseball game for ordinary consumers is not its commercial aspect.
.Org appears to be completely different in this respect, in that the primary association of the extension is with aspects of life other than commercial - cause, information, community, etc - all of which resonate well with primary associations of a generic term such as baseball game: information about baseball games, community of people who like baseball games, etc. And of course, you can have the commerce on the site like this, but the extension of the name correctly confirms that commercial stuff is secondary.
Even extension .net differs from .com in this respect. Generic terms often better fit the extension .net - as the latter does not contradict the primary association of the generic term with non-commercial qualities such as community of people, providing information etc regarding the generic term, but with added emphasis that this is the community of people, information etc over the internet. Just think about the website "photo.net". It is so much obviously better than "photo.com" - in my view precisely because the extension does not contradict the generic nature of the domain name, plus suggests the connection with the internet. Hence photo.net is a most popular community of professional photographers, which is at the same time a very profitable website - but they became so not by pushing on the commerce, but on features which in the eyes of ordinary people primarily associate with the generic word "photo" - information about photography, lessons, guides, community, competitions etc.
This how it seems to me anyway
But as I said, it seems anyway that the extension and how it fits the name is not the highest denominator in determining the professionalism of the website and the service it provides. Del.icio.us name was hardly pretty - but it didn't matter much because the idea behind the website and the execution was top notch, hence the deserved success and popularity. And then they were able to buy .com as well