From what little I know, all Shardblades share the following characteristics:
-They're usually (not always) very large.
-They can be a variety of shapes (I originally threw some really odd ones at Brandon, but he roped me in a bit and kept me from anything that was too scythe-like or had too many points on it, but there's a LOT of variation). It's not as nutty as something like Tite Kubo's "zanpakuto", but the concept that every one is unique might be considered analagous.
-They don't weigh much and are easy to swing around.
-They don't cut anything that lives, but they do sever all the nerve endings (not blood vessels though, interestingly enough) that the blade passes through. If they cut you through the neck or the head or the heart, you die instantly and your eyes smoulder and smoke out of your skull. Not sure if you could take a stab to the lungs (possibly, if it only kills one lung).
-They're able to cut through anything that is not alive, such as rock or furniture, with very little resistance. I don't recall that Brandon ever directly addresses the effect on clothing or armor being worn by someone who's cut, but I imagine that logically it severs that kinda stuff around the body, which could be hilarious if it didn't also kill and maim people.
-They can be summoned from nothingness, but it takes ten heartbeats to appear. This is not poetic description, the summoning is based on your heartrate, so faster heartbeats draw faster. They can also be dismissed almost instantly, disappearing into nothingness as well. They mist and smoke as they transfer either way, and when they first appear they quickly acquire condensation on the blade (suggesting that they are very cold, at least at first).
-Anyone who picks up the Shardblade instantly knows how to command its functions. If you die, your blade appears out of nothingness and drops to the ground. If you want, you may opt to discard it rather than dismiss it.
I think that's covers it... I don't know much about them that can't be read in the book. They're a fun magical weapon to imagine in action, and the design mandates allow for almost any shape and size Shardblade you care to imagine. Oathbringer is specifically described, but many others are not, and so any number of plausible designs are valid.
