Looking for feedback for a multiplayer online game. Is it okay to post a request for feedback on gamedev.net?

If you're looking for feedback on the graphics, ask in the Visual Arts board. If you're looking for design feedback, ask it in Game Design.
If you're looking for feedback on the story, ask in Writing. See how this works?
Moving this to the Lounge (it is off-topic in For Beginners).

I've found that the best way to get "feedback" is to actually have people play my game and to silently stand behind them and watch how they do. Watch where they get lost. Watch where they get confused or stuck. Watch how long it takes someone to figure out your puzzle or game mechanic. Watch when they laugh and when they cry. After they're done with the game, you can ask a few questions to get some insight into what was going on in their head when they were playing it. You'll mostly get emotional responses.

The important thing to recognize is that people won't be able to tell you what you need to hear. You have to see it for yourself, and you have to wash yourself of the stain of your ego, emotional attachments, and assumptions. I've realized that what appears to be an obvious, simple, elegant design solution can by really unintuitive for someone seeing it for the first time. I've assumed that leaving a big glowing thing on the side of a pathway "can't be missed", yet many people completely miss it.

So, yeah... get hundreds of people to try out your game. The less "expert" they are at gaming, the better.