I have been reading this discussion with some interest. (Why can people answer a question before being able to comment?). The answer so far is that new users might not understand the way the site works before commenting, thereby flooding the site with unhelpful comments:
New users might not understand how the system here works. They'd litter the place with "I have the same problem" comments (they do it in answers, too), they'd insult people, or some of the more nasty folks would post spam where it's very hard to catch.
We really want users to be trusted members of the community before they're able to leave comments everywhere. Enabling everyone to comment would certainly increase the amount of noise that we so desperately try to avoid.
However, I have made some posts on Stack Overflow (I have 175 rep) and therefore do know how the system works. I am not going to post 'Thanks', 'Me Too' or spam comments. It is consequently frustrating when I join another community such as this one and find that I need to gain more reputation to comment on it. Why can't I be allowed to comment across all sites once that 50 rep threshold is met on one site? Naturally this could also apply to some other privileges as well, and there could be other ways of doing the same sort of thing (only lowering the rep threshold on other sites, for instance). There have been other questions about transferring reputation between sites, but I can see that wouldn't work - so maybe this would be a better solution for these things.
(I did actually want to comment this on the other post but, ironically, I didn't have enough reputation on this site)
What do you think?