Check out the "file changes.txt" - there are quite some new items, inorganic, entity, creatures, etc. have all changed. A tileset like Ironhand that changes the raws would require some work to play nice with the new version.
In addition, some new objects (all the new book stuff) use tiles which now might look weird with Ironhand.
EDIT:
While we're at it, here's a list of the new objects and their default tiles:
scrolls: 236;
quire, bookcase: 240;
sheets: 245;
scroll rollers, book binding: 246
They all can be changed in the raws, except sheets, which seem to be hardcoded. At least I couldn't find their entry.
Note that tile #245 was unused before, and some tilesets might use it for something else. Also note that tile #240 is used for barrels/screwpumps, so that might look silly in some tilesets.