This might sound like an oddity at first, especially for those that are used to the limitations of meatspace, but it would actually be tremendously useful to have, as well as serving as one of the strengths of the Roll20 format. Often, players have to make rolls where they really shouldn't know the result themselves. For example, if a player makes a Knowledge Test or a Perception Test, they shouldn't know if they failed or succeeded - it should be assumed that whatever they rolled, that's what they see (or don't see). Currently, this is either glossed over, or the GM will have to do all the rolls himself, and check the player's sheets, adding a lot of needless hassle. The most annoying example I can think of is when you've got 5 players (or more!) and the GM feels the need to call for a general Perception Test. If it's done in the open (or by /gmroll), everyone that fails will generally know that they failed, while those that succeeded will know that they succeeded. If it's done by the GM, he has to sit there and roll each individually. With a /hiddenroll command, all the players could do their rolls at once, and only the GM sees the results, and can narrate or react to it appropriately.