I am interested in getting a sense for what people like about a "good" core mechanic. What makes a "good" core mechanic, anyway? What are some of your favorites from various games, and why? Have you ever like a core mechanics but disliked the system as a whole? Vis versa?
If you design your own RPG, I really, really, really believe the idea of mechanics is attached to two very specific things:
1. The mechanics will highlight the parts of the game you want most.
2. The mechanics will be framed by your own personal biases.
I have a third, but it doesn't apply to everyone, as evidenced by some RPG out there. But for me:
3. The mechanics should help build and enforce the continuity of the setting.
For example, when I sat down to create a system many, many years ago, I knew I wanted combat to be a bit faster than the Rolemaster, D&D, and Dangerous Journeys' games I had been playing. (That attaches to idea 1.) I also had a bias against abilities. I never liked the idea that the wizard couldn't control magic through physical strength, or a fighter wasn't book smart. It made no sense to me. So, I ditched abilities and just leaned into skills. (That attaches to idea 2.) Lastly, I knew I wanted a game where things like darkness, travelling, and exploration meant something. Therefore, I used the rules to help keep those things relevant.
This brings me to what I like:
A) Straight D20 rolls. Want mathematical complexity, have dice pools. But keep the addition and subtraction out of it.
B) Standards and levels set for important skill checks as opposed to a pass/fail.
C) Choices at every level.
As for liking a system, but not liking one of its mechanics - I can't stand the hit point bloat found in D&D and PF. But I like both systems.