The true key to it all, as someone posted upthread, will be how easy the whole thing is to houserule and kitbash into being the game each of us wants to play and-or DM; which will be slightly different for everyone involved.
If the core is kept loose such that individual elements can be modified/added/removed without too many knock-on effects elsewhere (e.g. removing weapon speed and weapon vs. armour type from 1e) it'll work. If the core is tied together so tightly that to modify/add/remove one thing here means you then have to modify most of the rest of the system as well (e.g. removing 90% of the skills from 3e) to counter the knock-ons, it'll sink.
My hope all along has been the core system for 5e will be very loose, with the various optional modules serving to tighten it up for those as wants such. But I'm not holding my breath...
Lanefan