Are there reputation or other limits on commenting on bicycles.stackexchange.com? Sometimes I see an add comment option, but only on the original question, sometimes below the answers and sometimes not at all. I am confused.
This is explained in the comment privileges page. You can find the list of privileges by clicking on "privileges" after clicking the little
v next to your name in the top bar. It will show you how close you are to attaining a given privilege, and clicking on one will give you details about it.
You can always comment on your own answers.
You can always comment on your own questions, and on any answers on your own questions.
Commenting anywhere else requires a reputation of at least 50.