I find ordinary glasses are perfectly good enough to keep out grit and insects. But two other features can be useful for cycling. First, they should have chromatic lenses, or whatever they are called, which darken in sunlight, because you can't be faffing around with sunglasses as well while riding. Second, rimless glasses are better for peripheral vision, especially at the top. If your head is slightly down when you are riding, the top of the rim of conventional glasses is just visible when looking forward. But with rimless, you will not be aware of the top of the lens.