Some glasses come coated in a substance that helps to prevent them fogging up. Usually, given that most modern glasses are well ventilated (often with small holes cut into the lens to allow hot air to escape), fogging should only be a problem when you come to a stop, but obviously you don’t want a set that completely mist up every time you have to stop at the traffic lights.
I’ve also ridden with a few sets of glasses that have a strap from the end of one arm to the other, and it goes around the back of the head to keep them on. In all honestly it’s pretty pointless unless you’re intending to use them for running as well, and even then a correctly fitting set of glasses shouldn’t really need a support strap.
If you need prescription glasses, obviously the last thing you want to do is ride without them. But for those who don’t use contacts, there are plenty of manufacturers who will offer their products with prescription lenses. It might cost a bit more and they’ll take a bit longer to get to you, but will be well worth it in the long run.
It’s by no means a deal breaker, but a good case to carry your sunglasses in always comes in useful. These come in all varieties from little pouches that also double up as lens cleaners, to full-on hard cases. The main bonus of a hard case is that it keeps the glasses safe when you’re not wearing them (especially helpful if you’re prone to standing on things or, more likely, you’ll chuck the glasses in a bag or suitcase when travelling).