Well, technically, we've always been able to return to dungeons. I've actually done that before in order to get an item I missed the first time, or just because I felt like fighting the monsters there again.
What may be added, in this case, is a scenario requiring you to return to dungeons to progress the plot. Making backtracking a part of the storyline rather than merely an option available to you. I personally think it's a great idea. I wouldn't mind finding new surprises in old dungeons after I haven't visited them for a while.
I didn't like the Temple of the Ocean King, but that was because it was the SAME dungeon, over and over again fairly close together in time. Not to mention the timer and inability to save your progress in that dungeon. If the backtracking is broken up over several dungeons, then it will remain fairly exciting and novel. I think Nintendo might have learned from PH how to implement backtracking in a way that appeals to gamers. Nintendo doesn't usually make the same mistake twice.
This could open up various possibilities. There could be rooms you notice the first time around, and you spend a lot of effort trying to reach them, only to find out you can't reach them until later. There could be multiple bosses and multiple hidden items in the same dungeon. This sort of feature could make the Dungeon Map and Compass even more useful items than they already are, because they would help you see where you've been already, and what you've already done/collected. With two or three different bosses and sections of the dungeon to beat, that would be needed badly. Although in some dungeons, perhaps they would only let you GET the Map in the second segment, to make the first one more challenging.
I hope that they also implement a lot of reasons to optionally return to a dungeon, like hidden heart pieces and such that you can't reach until later in the game. That would add a lot of replay value, and make the game feel less linear. Yes, this is one of the things about SS that I'm pretty sure I'll enjoy.