Choosing a Speaker Cabinet
Tone
Portability
Also, keep in mind weight and portability factors. An awesome oversized 4x12 cab with 3/4" plywood might sound nice, but can you get it to practice or the gig? Maybe a closed back 2x12 is more appropriate for your needs?
Similarly, does the cab have good handles, and is their placement comfortable for you? What about caster wheels - those can come in handy if you plan on moving the cab a lot.
Durability
This is hard to determine simply by inspecting a cab. You often need to rely on brand/model reviews from others. Cabs are generally made from plywood or MDF. The quality of the wood and the joints is crucial for the cab to last. Uneven edges, warped surfaces, gaps in joints, and flexibility are red flags. A cab should feel and look solid.
Depending on your particular circumstances, the type of grill may affect its durability. For most people a cloth grill will easily protect the speakers; but in more extreme situations a metal grill may be preferable. Of course, this is more than likely a customization you can make to a cab if necessary, not a deal-breaker. It may drive up the cost, though, so it's worth considering before purchase.
Aesthetics
Cabs generally use cloth or metal grills to protect the speakers. The metal grills have a few different varieties. The cloth are basically all the same but there is some minor variation. The cabs themselves are usually coated with a material to protect the wood, which can be a plastic material, leather (fake), or a carpet-like coating (just very dense and short). Sometimes the wood is simply stained and sealed. They are generally black, but there are other colors if that's what you really want.
Stereo Sound
Some cabs are wired for stereo operation. This might be desirable if you have a stereo power amp and want a mild stereo separation for your effects. Or you like to jam with your poor friend who has an awful cab, and you let him plug into yours.
The best stereo operation uses multiple cabs; however, spreading them wide for better separation. 2 2x12's might work better than 1 stereo 4x12.
Any cab with more than 1 speaker can potentially be rewired for stereo operation. Again, this is more about whether you want to take the time to do it or pay additional money for someone else to do it - something to consider before purchase.