Lots of things. Besides the bed leveling, the clean build plate, the right plate and extruder temps, you have to make sure that your designs are printable without tons of support and rafts and whatever else. Depending on what type of filament you're using, you might need to mod your printer with a cooling fan, or properly enclose your printer if you're using (dare I say it?) ABS. The 3D printer reality check is really just - experience (time and testing).