I do like the idea of having new areas to explore and more general pretty things to look at; it keeps vanilla from getting boring to the point of unplayability, for me. Though, I'd think they'd prioritize bug fixes over new stuff, since new stuff typically introduces a ton of bugs, only a few of which (usually the worst ones) get fixed before the next bit of new stuff comes around beginning the cycle all over again.
About the API, why would a modder bother with it? Because its included in vanilla? Unless they include modding tools, then I highly doubt anything Mojang could make would outclass Forge. Forge is already there, its firmly established its prominence in the modding community as pretty much THE mod API to use, and all by itself, fixes quite a few of Mojang's worst fuckups...like sending the whole texture library to the graphics card every tick, or in layman's terms, 20 times a freakin second. Forge is already the superior product; unless Mojang makes something that's truly mind-blowing, I doubt it'd matter much outside maybe having Forge wrap around some of it.
Honestly, the one thing I'd like the to fix is the absolutely moronic decision to have a static number of Item/Block IDs. Java has the facilities to make this dynamic. What would that mean for mods? How's something to the tune of 2,147,483,647 IDs for both blocks and items grab ya? All dynamically added, so the fewer things you have active in Minecraft, the fewer resources it takes up. It'd save so much RAM in vanilla, and it just...it just makes sense.
