I don't normally wear a helmet for road riding, but if I did I'd choose something that's well ventilated for (the summer at least) and that gave good coverage, based on the principle that no protective gear can protect what it doesn't cover. If there's a wide temperature swing between summer and winter, it might be worthwhile having two, or some means of insulating a ventilated helmet in the winter.

I see quite a few commuters wearing BMX/skate type helmets. It seems more sensible than some road-style helmets that are pretty much perched on the top of the head, with no protection lower down. The best protection would come from a full-face helmet MTB riders sometimes wear, but it's up to you whether you feel the need for such measures.