The comment link should be always visible. For users without enough rep to comment, clicking on it should give a nice popup, just like those on the vote buttons, informing them that they need more reputation to comment.
What is meta?
- Any Stack Exchange user can ask a question
- The community provides support, votes on ideas, and reports bugs
- Your voice helps shape the way Stack Exchange operates
I suspect that will generate a different frustration: Folks will complain about why they're encouraged to "comment" if they're not allowed to yet.
But I agree with the sentiment - in a more "button-based" app, I'd think what you'd want is the equivalent of a "grayed-out" button in a GUI, possibly with a hovertip telling you how much rep you need. Would that work in a "link-based" app?
Remember that users with 1 reputation can comment on their own questions, answers, and any answers to their questions.
In fact, we pre-expand the comments on questions asked by new users to ENCOURAGE them to use comments.