Everything:
Engine cradle and torque arm. The torque arm ties the rear differential to the tail of the transmission, replacing the mazda power plant frame. There’s some debate as to the design (some suggest that the differential should be tied to the chassis instead). Supposedly Mazda’s power plant frame reduced driveline slop by transferring any loadup on the engine/transmission mounts to the differential, but at the same time, people seem to crack the stock power plant frame. Regardless, this one is tubular steel, and probably won’t be breaking anytime soon, knock on wood.
Engine cradle was spray painted, not powdercoated as requested. That's not the problem though. The problem was that the subframe wasn't actually welded together. It was tack welded, and then spraypainted. My understanding of the proper procedure here was to do a full weld, then grind the weld, then powdercoat the piece. It seems that somehow...that didn't happen.
Coming back to this post several years later, the engine ended up almost falling out of the car at an autocross event. I limped it back home and replaced the front subframe.