There's been a lot of nice quality of life changes in the 3.7 builds (which has now become 5.0.0) that make going back to the older versions a bit painful.

Also some pretty major gameplay and balance changes, some of which are pretty controversial. But overall, I think that it's a big improvement, and although I don't necessarily agree with all the changes it certainly makes the mid and late game a lot more interesting and varied (not to mention dangerous) than it was in 3.6.7.