You only need 50 reputation to comment anywhere on the site. That's only 5 upvotes on a good answer or 10 upvotes on questions. It's an extremely low bar to get over.
Note: You can always comment on your own posts (questions or answers).
It's a shame that that's kept you, and probably others out. Honestly I think it's worth it to have some small barrier, but whether it's helping much is debatable- there's still been a pretty bad downturn in the quality of content in some areas over the past couple years.
The 50 reputation barrier only stops you from commenting. It's a spam prevention measure. Anyone can answer a question (even anonymously) with 1 reputation. Anyone who registers can ask a question.
It's extremely frustrating when you want to comment on a different Stack Exchange site but can't due to having no reputation... despite the fact that the new account you made is 'linked' to one on Stack Overflow with thousands of rep.
If you have thousands of reputation on SO then you should get the 100 point bonus on every site where your accounts are associated. If you don't have it, open a support post on Meta (http://meta.stackoverflow.com/questions) and we'll try to see what's up.
I only have 477 points on SO, yet when I sign up for other stack exchange sites I immediately get 100 free points. The bar really isn't that high. It's just designed to force newbies into those specific activities that one ought focus on first: providing good answers to questions, and asking good questions.
Note: You can always comment on your own posts (questions or answers).