I have the most fun starting the next "phase" of my building. When I start a new world the very first thing I do is set up Tinker's Construct and some iron/gold chests for better storage. (I play Infinity) Once my "smeltery phase" is nearing completion and I am looking towards my next project, this is where I have the most fun. Usually the next "phase" for me is setting up basic ore doubling, quartz grindstone, then sag mill and other basic enderio machines. After that I enter my "energy production phase" with big reactors, solar panels, and the likes. I think it is the satisfaction of completing the previous phase that I really enjoy.