To encourage feedbacks to your game, you have to demonstrate how committed you are to its development. If you just post a link to your dropbox, you are cutting a lot of the potential feedbacks people could give.
Describe in few words your game genre, view, mechanics and universe (e.g. a 2d side scrolling platformer featuring a plumber, on a quest to save the princess of a kingdom populated by evil mushrooms and turtles). You could also describe the scope of the game (8 worlds with about 10 levels each and some secrets ones, for an average of 10 hours of gameplay), the number of assets (types of enemies, power-ups...) or some other outstanding feature to make us feel your game is unique and interesting!
People like pictures and posts with pictures get much more comments than the ones without. Post some nice screenshots, gifs (check lhttp://www.cockos.com/licecap/), or an embedded video of your trailer and latest gameplay video. You should update these often, visuals are the main appeal of this page!
Tell us on where you plan to release your game, whether on pc, online portals, consoles, mobile and maybe the specs you envision. If you go with self-constrained development (CGA, 16 bits, 5 events...) you could also tell about it beforehand.
If you didn't introduce yourself in the presentation thread (you should), and even if you did, a few words about you , your strengths and your background would help to give more accurate feedback. If you are more than one, it's a good place to give credits to the awesome people working with you. And if you are still looking for someone to fill in a position, it's also a good place to say so!
It can be useful to specify what you consider to be done, what is in progress and what kind of feedback you're looking for. If you want people to comment on something specific or to ignore something that is not implemented during their playtest, you should indicate it as well.
The link for people to play the latest build of your game
Here you go! Remember that this is just a template and you should modify it to suit your game. The first post of your devlog is the place people see first, so you should update often if you got any progress.
Be open to bad critics when asking for feedback, people reply because they care!