1. Flat-chested characters will be censored: Niel figure is different, not brainwash-able (also not required to be brainwashed for endings). Vivi won't spawn. These two won't be in gallery either. New-born females will turn into grown forms skipping loli stage.
All of that can be restored with "Patch", so don't worry about it. It's just how it had to be done for Steam.
2. Rune ending requirements changed: build 10 rooms (was 50 rooms before) and sell 100 eggs. Done for balancing - easier to get without building tons of rooms. Getting her ending still results in game over... well, I still think that's a canon ending for MBM.
3. Remember that white character outline? In old version each object have 3 textures: main, black (opaque) and outline-white. In new version there's only one main texture and outline is done with a shader. So no more additional GPU compute for texture duplicates = faster performance. New method looks a bit different, though.
4. Since there are less textures and less GPU memory is used they upgraded texture quality. Original release used a mix of DXT5/DXT5Crunched compression for characters. New version use BC7 compression. Unless you're trying to emulate it on ARM or playing on 10 years+ old hardware it's a preferable format.
Mind you, it's not a tremendous difference in quality, but art is just not suffering from additional compression:
View attachment 4838950
5. Data structure is different, easier to mod character assets and no need for constant packing-repacking procedure. It still requires Spine software for a proper editing. Don't expect big breakthroughs in modding either, it's not built for modding in mind like ProjecR is after all. Most of stuff is still hard-coded.
If you want my opinion, new version is worth it from a technical standpoint. I'll make sure to adapt my mod for it, there's still lots of CPU performance headroom available if I'll optimize character mesh complexity. Tits mod won't be supported on release since I'll need to adapt its files to work properly on steam build. Old BepInEx (ModPack) mods should work fine, though (ComplexBreeding should work as well).
Can't say anything more, since I simply don't know what's going on with Steam Review process *fingers crossed and all that* You should wait for news from devs about that.