For those of us for whom gushing doesn't come naturally, though, it can be harder to think of something to put into a comment box. Coupled with the fact that it comes across as rude to leave something very very short (even if not necessarily to the author, to other readers), this leads to me finishing reading a story, and then leaving tabs open for days and days while I reread and try to formulate some thoughts.

Sometimes it's easy, because there's something I immediately liked about it that I can describe. (I try to give an impression of general mood, plus at least one specific thing I liked.) Sometimes... not so much.