FWIW, Warby Parker was not the first web-based retailer to undercut traditional eyewear sales, nor are they the cheapest. They do seem to have the best PR, though.
Excuse my ignorance, I haven't even heard about Warby Parker until this article. How long have they been around (my quick Google searches have failed me)? I have heard of ClearlyContacts.ca (I am Canadian), is Warby Parker comparable?
They basically started a very customer friendly model that other online glasses retailers didn't offer at the time of founding.
EDIT: Basically, customer experience.