Active or passive? Most sound bars are "active," which means they contain all the amplifier and signal processors necessary for stand-alone operation. However, there are some "passive" sound bars that rely on a separate A/V receiver or amplifier to handle those tasks. If you don't have a receiver or amplifier, you definitely need an active sound bar.
How are you hooking it up? One of the most important factors in choosing a sound bar is what you want to hook up to it and how. The easiest way to connect a sound bar is to plug all your other equipment into your TV and have a single cable connecting the TV to the sound bar. However, that requires you to use the TV to switch between different inputs -- with the catch being that not all TVs will pass full 5.1 surround (only stereo) via their optical output (see this article at Big Picture Big Sound for more information. If you don't want to do that, you need a sound bar with enough inputs to connect to all your other gadgets - analog inputs for older equipment, and HDMI inputs for newer gear. Other inputs that can be useful are USB, for hooking up an iPod or a Flash drive, and Bluetooth connectivity for streaming from your phone. Pay attention not just to the number and types of connections but also to where they're located. Some sound bars put the inputs and outputs not on the sound bar itself, but on the subwoofer, which limits where you can position it. Others put them on a separate box, so neither the subwoofer nor the sound bar has to be tethered to your TV.
Where will you put it? In addition to working with your other electronics, a sound bar needs to fit in the space you have available. If you're putting the sound bar on the TV stand, make sure you have room for it in front of the TV set and that it isn't so tall it will block the beam of your remote control or obscure the lower edge of your screen. If you choose a sound base that sits under the TV set, make sure it fits on the TV stand and is big enough and sturdy enough to support the set. If you're wall-mounting your sound bar, choose one that's about the same width as the TV screen; it looks better, and the position of the sound will match the image on the screen better. If you're buying a sound bar with a separate subwoofer, figure out where you're going to put that. A subwoofer that connects wirelessly to the sound bar will be easier to tuck into an unobtrusive spot.
What will you play on it? If you're using the sound bar only for TV watching, even a basic sound bar with 2.1 channels – two speakers in the sound bar, plus a subwoofer – will be a big improvement on your TV's built-in speakers. However, if you're planning to listen to music as well, it's worth investing in a higher-end model with better sound quality. If you enjoy music or movies with a lot of low-end sound, such as high-octane action films, a separate subwoofer is a must. And if you want a true surround-sound experience, you should look for a sound bar with both a subwoofer and separate rear speakers rather than relying on the digital hocus-pocus most sound bars provide.