Good morning!
1) how common are comms failures?
That entirely depends on the aircraft you are flying and it's maintenance. And how much redundancy is built into it (dual radios, dual alternator, ...). I have had two radio failures in about 3000 hours on light types, both were caused by total loss of electrical power.
2) If I do get one, what features am I looking for.
Transmit and receive I would say.
The cheapest (certified) unit will do the job. A long battery shelf life is nice to have, so you do not need to think about charging the unit too often. All the other gimmicks (e.g. VOR receiver) have long been obsoleted by our GPS equipped smartphones or tablets.
Personally, for flying light types at low altitudes (say below 5000ft AGL) I do not carry my handheld radio any more. The cellphone is good enough for calling the control tower, just make sure the numbers of frequently visited airports and airfields are stored.