From Steams own page
"This has been coming up more and more lately, and seems to warrant discussing our perspective on the practice of giving away copies of your game (either directly or as a drawing) in exchange for votes.
When you give away copies of your game in exchange for votes, you put us in a really uncomfortable position. We do not think these votes accurately reflect customer interest and it makes our job harder in deciding which games customers would actually buy and play on Steam.
Additionally, when you give away copies of your game for votes, then every other developer on Greenlight thinks that is now the thing they need to do in order to get noticed. We don't think that is healthy for the system or really what customers want.
We understand that running contests or giving away copies of your game can be viewed as a form of marketing. But for the purposes of Greenlight, we don't think that giving away copies of your game in exchange for votes accurately reflects genuine customer interest.
This is something we continue to take into account when evaluating titles to be greenlit. The result is that it may take significantly longer for your title to get Greenlit, as it is much more work for us to try and understand customer interest in a title that has collected some unknown number of votes in this manner.
Reminder of Submission Guidelines
The Greenlight FAQ is here, with general guidelines: http://steamcommunity.com/workshop/about/?appid=765§ion=faq
As a general rule, your submission needs to have at least a gameplay trailer, 4 screenshots, and a written description. The 'gameplay' part here is key. If your game is too early in development to show what the game is actually like to play, then you have a concept and not a game. Please post as a concept until you can produce a trailer of in-game gameplay.
We hope this all makes sense and sounds reasonable. We are continuing to work on improving our distribution process, but there is significant work remaining. In tnhe meantime, Greenlight is the system we have to work with."