For me it's time...I'm lucky if I get to brew once a month due to a crazy schedule and that of 2 young kids, so yes, I have this dilemma every time I ponder what my next grain bill will be. Sometimes though, it gets overwhelming just trying to figure out what to brew next, so today I just brewed a SMaSH for the first time. It was kind of relaxing to only have to measure out one grain, and one hop strain...
Packaged: Fruitcake Old Ale
Primary: Simple Cyser '15
Secondary: Why do I keep this line here...?
Bulk Aging: Cocobochet, Wild Cider (2014 & 2015)
Planned: So many ideas, not enough time to brew them....