Above all, make sure you get one that fits you (or that you can adjust to fit you). When pedaling, your legs should be almost fully extended, just a veeerry slight bend at the knee. If it's too short, you'll be working too hard. You shouldn't be able to have both feet flat on the ground while in the saddle. Do make sure you can stand (off the saddle) flat-footed on the ground without the top tube banging you in the privates (I'm assuming you're a guy). If they'll let you, test ride it and run it through it's gears to be sure everything's working fine. Stuff can be fixed, but I'd rather choose an already working bike over one that needs work, all else being equal.
Next, I'd look for big, medium to skinny tires. 700c x 35 is what I have. I've found out that a bigger tire vs a smaller tire (700c vs 26") means you go further with each pedal stroke. And skinnier, smoother tread tires (that are inflated to higher PSI) are better for going faster on pavement. Fatter, smaller tires are better for off-roading or riding on poor pavement with lots of cracks, potholes, cobbles, etc. The knobby mountain bike tires absorb some of that roughness, but it tends to be more legwork.
And get a U-lock. Cable locks are suuuuper easy to cut, and you don't want to come outside to see your only transportation gone.
