I love possibility to play in MMO Fallout - few years ago it was even unimaginable. Also freedom - you can play as crazy badass with minigun, as shitshoveler, humble drugmaker, loner with tent and few caps in pocket, brave Spartan warrior, even NCR dweller standing whole day next to trash bin. Imagination is the limit.
I hate limitations like cooldowns, timers, level cap, everything that restricts your freedom and forces using alts, fast-relogs or other crap.