Another great resource is the Epic Games Forum. Go to the modding/level creating section for the specific game you are making a map for.
Some advice for people who are making maps for UT3 or GOW and are thinking about posting it on the epic forum. Make sure that you understand the community that goes to that forum. They are very different from the people here. People there are looking for gameplay when it comes down to it. Yes a different environment from the shipped levels is appealing to them... but they won't like the map unless it is fun.
So here is a basic timeline of the posts you have on the epic forum when you are sharing a map. This may seem stupid to some people... like "do you really need to follow this?". My response is that it isn't necessary... but it something to think about. Devoted Testers are hard to come by and if you overload them with releases... they won't bother downloading them and putting time into playing it, getting an opinion, and posting their thoughts.
Post your Alpha release. This should be not much more than a BSP. No static meshes are necessary here. Test it with bots and make sure there are no clitches and that someone else will have fun playing it. With that said, if you have a friend that is willing to play it for a little while... send it to them. That can be really helpful so that you know that you aren't the only person who enjoys the map before sharing it with the internet.
Listen to what people give you for gameplay feedback. Make sure you don't post another version of your map within a week after your first release. Chances are that you aren't going to get too many devoted testers who are going to want to test every little change. Posting a new version everytime you change something like elevator speeds will make them not want to download the new version. Then when you release the final... nobody will see it.
Post a Beta. Your beta should take a sometime to nail that core gameplay. You should have listened to everything people said about your map and really act on what they said. If someone sees that you didn't address what they said needed work... well... you probably just lost a tester. Make sure that your beta really nailed that key gameplay element that makes your map special. Your Beta should also have some life. Even though you want to wait a little bit before posting your beta... I personally think that your beta shouldn't include all of your custom assets... this would take longer than you want and people will forget about the map. The gameplay should be considered done. Now I'm not saying that your Beta should be the default checkered material. Take sometime looking through the assets that come with the game to give it some spice. After your beta... the only thing left to do are the custom assets and ART... yes ART. If you are a polycounter, chances are art is very important to you. Make sure that after you post your beta you start working on your assets. It's going to take a long time to create all new assets and you should get started as soon as you feel comforatable with the layout. I would suggest starting with assets that are going to be in a section of the map that is unlikely to need tweaking. This way you don't waste your time making assets that don't end up getting used.
Same as the Second Post(s), just listen to what people have to say. If you don't get any gameplay critiques then just keep working on the assets. But if you do get a lot of critiques that aren't necessarily "minor problems", consider releasing a second beta (BETA2/B2) that address's those problems.
Post your final release. Like I said above, this might not be your 5th post... but it is the fifth step in my mind. There is going to be a pretty big gap between the Beta and the final due to all the custom assets you have to make. In that time you should start a thread here and to get art critiques if you haven't already made a thread. There really isn't much to say about your final release... except that it shouldn't be rushed... especially considering you will be doing, what I hope, is your true passion... art.
So that's all I have to say. I know this might not be a perfect workflow but I think it will help get the most out of the people there. Oh and please don't think that I'm classifying them negatively... I'm just saying that the people here (on polycount) are focused on art while the people on the epic forums are less focused on art and more focused on gameplay.
There are a lot of people on this forum that have more experience than me so maybe some people could add to what I wrote.
I also don't know if this is the best place to post this but it seemed like a good idea while writing it. :P
Hope this is helpful to some people.