Size, warranty, reimbursement if your system gets fried are all easy to see on the box. If you want to get a bit more technical, then try and look for these items:
Total joules dissipated in the event of a surge. How much overvoltage can it ground before it has to let some through. No - it won't totally ground out a lightning strike, just part of it so be realistic. The higher it is, the more likely it can stop normal overvoltages.
Does it have EMI/RFI filtering to reduce line noise? Noise on the line causes strange hiccups on your system - corruptions, blue screens...
What is the waveform? Normal power is supposed to be a sine wave. Cheap UPS devices will create a square wave. The quick ramp up in power tends to overheat your power supply and the quick drop isn't so good either. Better UPS's use a modified wave like a trapezoid wave and the best and most expensive almost duplicate a sine.
Does it cut over to battery on brown outs and overvoltage or does it modify the power? If the UPS is better quality, on brown outs, it won't completely switch to battery, it just sips from the battery to make up the difference. On overvoltage, it tries to clip the surge.
There are other specs, if you are really interested, go to the various websites and look at their top of the line models to see the features.