I always end up with one or two missing slabs (or tiles or platforms, if you prefer) in the Holy Sanctum in the 4th act. Is it possible to get all the slabs, and is there any reason to do so? Bonus loot? A feat of strength achievement?
It is possible, but no, nothing interesting happens.
I completed all the slabs on my last run through this dungeon (Hell difficulty), and absolutely nothing happened other than the mobs that spawn when you step on a new slab.
Partway through, I managed to get to the far corner, where there was a Resplendent Chest waiting for me, but it definitely was there well before all of the tiles were filled in.