Reading his response, it seems unlikely that they will be implementing weight, as these would add an additional value to items, which bogs down inventory management almost as much as the 'tetris inventory'.A weight system is simply a different approach to inventory restrictions, and it's actually fairly similar to a grid based system except that weight systems are generally augmented by a character stat instead of being item based upgrades.
The main issue with these systems as they relate to Diablo III is they add an additional value to items. That secondary value works to complicate and thus slow down the inventory management of a player, drawing their attention away from the action, which is of course the main focus for us with Diablo III.
I've played a few RPGs with weight systems, and they're among my favorite games of all time, but it's a case of choosing which systems works best for each game.
I wouldn't be surprised if they used World of Warcraft's system, perhaps with a few interface tweaks that allow for simpler management of equipment, as some player-created mods for World of Warcraft offer.