I've heard a lot of speculation on the short/long rest being one of those 'dials' that will be adjustable in the DMG. Some have spoken of Long rests being moved to a once a week, or only in a safe haven type of thing, while the short rest becomes a night spent roughing it in the wilderness/dungeon. This is a wonderful way for each group to adapt the mechanics to their particular play style.
The problem arises when the ratio of short to long rests changes with the dial. If everyone has to high tail it to Rivendell or Lorien to get the benefits of a long rest, then those classes that get more out of a short rest are going to be a lot more effective than they would in the default system. That is unless the DM is very careful to also dial back the number of encounters between long rests, it could make a very noticeable difference with certain classes.
The problem arises when the ratio of short to long rests changes with the dial. If everyone has to high tail it to Rivendell or Lorien to get the benefits of a long rest, then those classes that get more out of a short rest are going to be a lot more effective than they would in the default system. That is unless the DM is very careful to also dial back the number of encounters between long rests, it could make a very noticeable difference with certain classes.