Poll results and new mock-up screens: http://edmundm.com/post/37223982057/...results-are-in
Also, Edmund divulged new details in a recent interview
-Nicalis will first remake the whole game (base Isaac plus Wrath of the Lamb) then Edmund will step in to add the new content.
-16-bit pixelated art style is here to stay. The old look will not be included.
-Isaac's pixel look currently a work in progress. His head size will actually be bigger because of the limited amount of detail the pixel look allows.
-Arcades will be made to be less of a time sink. Edmund is considering making slot machines explode more often.
-You will encounter new items that are sensitive to time, which will get players to finish a floor sooner.
-The new extra floor seems likely to appear after Sheol. The choice between going to Sheol or the Cathedral after The Womb/Utero will be made more interesting.
-There will not be "custom" challenge runs. However, there will be a character that touches on the concept of them.
-No in-game guide to tell you what items do.
-New secrets that will be hard to find.
-New "spider love" item that works similarly to the Skatole, only for, you know, spiders.
-Smooth, steady frame rate is a priority.
-Really crappy items like Lemon Mishap and Poop will be buffed. In the case of Lemon Mishap, its creep will stay onscreen longer.
-Very, very likely no new items that enable your character to fly. Edmund admits he overdid that with Wrath of the Lamb.
And in case you skimmed over it in the poll results post:
-The ability to save and resume your game will be in Rebirth.
-There might be a feature to screenshot your final look when you beat the game.
-Local co-op confirmed though there might be the possibility of drop-in support as well.