I agree with OP as I've experienced this exact issue myself.
I wanted to raise the profile of a question that was unanswered, an and exact duplicate of an issue I was having. I could neither upvote, nor comment on the question. [Note: I can see the value in not allowing new users to upvote/downvote]
Another time I wanted to comment on an answer, and I was unable.
Some here have claimed that it's easy to rack up 50 rep, but it isn't when you a) Have no original questions that haven't already been asked / aren't easily google-able, and b) Aren't knowledgeable enough to answer most questions (This applies to me on SO), beyond any answers they may have already received.
I have found SE one of the hardest communities on the internet to actually participate in any meaningful way in.
Perhaps, instead of lowering the required rep, giving new users an allowance of say x-comments per 24 hours. For people like me, who want to improve, and want to give back, but find themselves unable due to lack of knowledge and lack of new questions, that would be a nice bridge between the 'I feel as though I can't actually do anything beyond read on this site' and 'wow I've thought of several questions. [Several hours later] Now suddenly, I can vote, comment and actually take part'.
For some SE sites (notably SO and Programmers.SE) it is still a very static read-only experience for me. The level of questions are far above my current level generally, but reading the answers I often want to comment on the question or answers, perhaps to ask a tiny question about a very minor detail [that would normally be closed as too specific] about the code sample provided as an answer. Any new question I ask, at my current level, will be either googleable or already asked [Thus not really helping me gain rep - as It'll be closed as duplicate or easily google-able]. But if I could participate in comments, even with a limit as I suggested earlier, it would help me improve far more.