I'm new, but have already seen where a "possible answer" I posted, which started with a clarification question, got me a "down vote" for making a "not useful" post. So today I saw a question where I thought I might have a solution, but it was not clear what the poster was trying to accomplish (he said "control" some motors... but control as in 'speed', 'direction', or simply 'on/off' was not specified. Not wanting to make the same mistake twice, I tried to click on the "comment" link, to ask for this clarification, before venturing a possibly "not useful" answer. But I wasn't allowed to comment. Does this make sense? Or did I simply miss a better way to ask for a clarification?
Yes, clarification questions should be asked as comments, and the commenting privilege comes at 50 rep. Don't worry though, you'll accumulate the needed 50 rep soon enough.
In theory, it's also possible to talk to the asker in the EE.SE chat. But that means that they should sit in the chat room for you to talk with them.
It's understandable why this exists, but it also has a fairly negative consequence: A new visitor who wants to ask for clarification on an existing answer has no choice but to break the rules, either by:
1) Posting as a new question, which will probably be closed as a duplicate
2) Posting a question as an answer, which will be downvoted and flagged as improper.
There really isn't any "proper" path available to them. And please don't say they should hang around for a few days trying to accumulate points first - we don't require that for other questions. As it is there's almost a disincentive to research before asking, because someone who does not and just posts blindly can at least claim ignorance of the duplicate.
Note that recognizing that a problem exists with the current mechanism is not the same as saying it has no benefit.
(There is one other escape - for someone established on another SE site, linking a new account here will instantly get them over the barrier)