I really like when they decouple skill/class progression from level ups. Like Dragon's Dogma, you have XP and Discipline, and a separate class rank from your main level. Way more interesting than Diablo/Bethesda/Bioware style of streamlining everything into a single level progression. More ways to level up really adds to the depth and makes me want to grind rather than feeling like a chore.

I also like systems where you use XP to buy upgrades, like the Rad Codex games (Kingsvein, Horizon's Gate, Voidspire). You can work on lots of different boosts and upgrades without just one boring level grind.