A minor but beloved bug was removed: players could no longer sneak-jump up 2-block heights by holding shift at weird angles. This made cliff scaling slightly more realistic and much more annoying.
In a way, Alpha 1.2.7 is the unsung hero of Minecraft history. It asked for no praise. It simply worked—or worked well enough—and for Alpha-era players, that was everything. minecraft 1.2.7 alpha