0. If someone I know and have a good opinion of is making the game, I'm much more likely to pay attention. This isn't necessary, but it helps a lot. I mean, I don't really even go into PD anymore unless I know someone in the IRC channel is making a game or I see a link in the sig of someone I like.
1. The thread has to not look like shit. Bad grammar, poor formatting, etc. make me leave immediately. If your presentation is good, I'm much more likely to stick around to see what your game is actually about.
2. Good screenshots. RTP is an instant turn-off, but if it's used well and doesn't look exactly like every other horrible RM game out there, I might stick around. In addition to poor mapping and etc., if your screenshots display horrible dialogue, loads of amateurish Photoshop gunk, or LARGE ANIME PICTURES, I'm gone. More than anything, I can't stand bad writing, but if your graphics suck, it probably all does.
3. Originality. If you pass the first two tests, I'll take a look at your premise. I don't read long plot descriptions, and I certainly don't read essays on the entire backstory of your world, but your project needs to not be generic fantasy trash. I also like to glance over the characters and see if you're using a bunch of archetypes. Basically, if your writing is shit, I'm gone.
4. Gameplay. I like project threads that make it clear that you have cool mechanics, good dungeons, whatever, and moreover that you're making it work. A list of features isn't impressive (and in fact turns me off because it looks like you're loading every script you can find into your game regardless of whether it's relevant); you somehow making it clear that whatever features you have work together and make your game fun to play is. If you can do that and meet the above requirements, I'm sold.
Basically, the key is to make a game that doesn't suck and have a project thread that makes this clear. There's no way to get around needing a good game: glitter doesn't make shit stink less. But that said, there are some things you can do to make your thread more eye-catching, which I believe Volrath wrote an article about once. I recommend that, and I have two tips myself: 1. have a positive reputation around the forums and 2. make your thread focus on what's special about your game, making sure it shows off all the good design and demonstrates how your gameplay/story/whatever is special, preferably with lots of shiny images. And I mean images that relate to the game, not section headings with Photoshop filters applied.