There are some really good posts in this thread. I never considered the quality of extrusion to be what makes one rim "feel" easier than another to build up with consistent tension, but this makes a lot of sense. What I have noticed, as has been mentioned, is that between different rim manufacturers, certain rims build "easier" than others. Of the wheels I build, Mavics are easiest to build into consistently tensioned, true wheels. Alex and Weimmann are more and more "difficult" respectively, requiring more time to achieve an overall lower quality wheel. This isn't an endorsement of Mavic necessarily, just my experience churning out about wheelset a week for single speed (internally geared) commuter bikes.