Most obviously - things that would no longer work in 3, that used to work in 2 - if that was not a problem, then it would've been an issue to switch (and the pe

Most obviously - things that would no longer work in 3, that used to work in 2 - if that was not a problem, then it would've been an issue to switch (and the performance is not worse than 2). But I guess it's not that easy. Also the "C" api must be backward-compatible.