-the body style & shape: the shape will affect how the guitar feels in your lap, the size of the body will determine the guitar's over-all volume. For instance, dreadnaught acoustics that you see a lot of country guys use have a lot of volume... great if you're going to jam with other people, but maybe not so good a choice for those practicing in a thin-walled apartment.
-construction materials: in your price range, you're probably going to seeing a number of options that use "laminate" and other artificial materials (not real wood) to build the guitar & neck. This is not necessarily a bad thing. I find that guitars made from laminate or essentially plastic "fake wood" tend to sound quite bright, lacking the deeper tones and sonic breadth that real wood guitars have, but still sound really good, and the build quality can be quite high. Give these guitars a good hard test-drive, and if you like one, go for it. Real wood is nice, but be wary of "real wood" guitars that are super cheap. They might be real wood, but the construction may be pretty poor and components like the nuts that keep the strings in tune may be crappy. My buddy has a cheap acoustic and its impossible to tune properly. The intonation is awful (how in tune the guitar is with itself).
Going used will get you more bang for your buck, and especially with acoustics, there are always great deals on Craigslist to be had. Just look at an add, then do a Google search to get an idea of reviews and prices for that guitar.
Finally, since this is your first guitar, as hard as this is, let go of caring about how the guitar looks. If the guitar is a hideous color, has scratches or maybe a chip out of the headstock or something, you can save yourself a HUGE amount of money. If you get really deep into guitar, you'll be wanting to spend 4x as much as what you're budgeting now, so you'll likely be letting this guitar go in the future anyways.
Don't forget to go into a place like Guitar Center and try a whole bunch of acoustics. Ask to borrow a tuner when you go in, and be sure to bring a pick. Don't be afraid to try out some $2000 ish guitars just to see what they are like (don't drop it!). You might be surprised at how well some of the more affordable models stand up.
Happy shopping!