RPG mechanics that I really enjoy:

1. Alchemy: I find that I really enjoy mixing ingredients to create equipment. For this reason I love the Atelier games, as well as really enjoyed Dragon Quest VIII.

2. Customization: Let me create my own main character, I love assigning stat points and customizing my character's physical appearance. And if changing your armour actually alters your sprite/character model even better.

3. Character collecting: Maybe it's just because I grew up on Shining Force, but I feel too few RPGs give you the opportunity to really pick your own traveling companions. This was a common staple of the 8bit gaming era that has fallen to the wayside. What would Suikoden have been without 108 possible party members?

4. Mission Quests: I love journals. Let me stray from the main quest with a checklist of a thousand possible objectives to complete. Maybe it's a for of OCD but striking off a page of completed tasks is really its own reward.