Unlike MC 1.7, where stepHeight is set in EntityLivingBase to 0.5f, in 1.8 it's set to 0.6f. The EventHandler class's onEntityUpdate method added in v2.0.0-17 sets the step height to 0.5f, which causes stairs made with Chisels & Bits to be unusable. Although vanilla stairs/slabs still work, I'm sure many other problems will arise from reverting player step height back to the 1.7 value.

b5c8c22
That's a weird issue, btw. Why'd they change the default...

Afraid I haven't a clue. Sure, step height is only referenced in moveEntity, and they did rewrite it, but I have no idea if it had to change because of that or because of some change deeper in the code.