Since combat starts immediately, and your moves are randomly displayed on numbers 1-5 there's a bit of an issue of getting suckerpunched. If a mob engages you at short range, they can get a hit off before you even have time to gauge which key to hit for a defense. Mostly this is a problem with hunting, but I could see this being an issue in pvp too.
Is it worth considering an Omni block weighted towards your highest combat stat that lasts for 5 or maybe 10 seconds when combat is initiated with you? It would give people enough time to get their bearings in a fight without being instantly slammed for damage, but since it's combat weighted it would only really absorb the first attack or so.