Actually, I disagree to an extent. Even a "Good story, keep it up!" review can provide a lot of motivation for a writer. It's certainly easier to garner hoards of reviews if you write in one of the more popular fandoms, or if you follow the more popular trends in Original fiction. For me, that just makes my few reviews the more wonderful.
Some of my most cherished reviews are the wonderful concrit my dear friend Shadowknight left for me. Sadly, real life has him far too busy to come and play with the deviants here, but he was a treasure as a reader and reviewer. However, the reaction from some authors, who delete reviews they think are unfavorable or reflect a lack of understanding on the part of a reader, will cause reviewers to stop reviewing.
If you take the time to leave a review, it means the author got a response from you. As an author, that's what I want. I want to know I got to you, good or bad. I also want to grow as a writer, and knowing what I did wrong is as valuable as knowing what I did right.
To touch on what LockedBox said, creating a thread to communicate with readers is a great way to build a loyal base of readers who'll review you. Readers love getting to know an author, and they might be more candid in a discussion thread than on a public review board as well.
Scribophile is a good workshop/writing group, but it's not really an archive, since non-paying members can only have two works at a time posted there. However, if you want to improve as a writer, it's a great place to belong. You actually need to review/critique to obtain points before you can post, so the entire site is involved in the review and critique of the submitted works. It's plentiful feedback, but again, it's not an archive. At some point, you'll need to either pay for membership, or move your reviewed, critiqued and polished works to an archive.