Pretty much what the Roudmeister said.
You can get Improved Critical, in effect, from the Soldier Advanced Class, and it's fairly deadly. To nitpick, I wouldn't specifically say that Massive Damage takes the PLACE of Improved Crit, but I'd say that they're pretty dangerous together.
In the same vein, so are weapons with x3 damage multipliers. I'm running a Grim Tales game, and man, when that axe-wielding mook rolls a 20, it's pretty much a guaranteed Massive Damage check -- d8+3 becomes 3d8+9 in one fell swoop.