I should like to open the thread which will be collected every possible feature that you can implement in a character rig.
It would be great if the feature will be on a par with a brief description, if it is not widely-used.
If the list is not full, you can continue it…
Every possible feature is kind of a big thing. Also every feature imaginable would be impossible for any rig.
However, the features you mentioned here are standards. But what I like in a rig is features to aid against counter animating. That’s really the worst thing I know. Of all the free rigs out there, hardly any comes with a solid foot roll(like the Jason Schleifer one) where you can specify how much it rolls before standing straight again.
So to build up on your list. What I want to find in a rig would be:
5. Reverse foot rig with attributes for ballroll and tip straight.
6. Auto-clavicle and auto-scapula that follows geometry.
7. Bendy limbs.
8. Proxy rig
9. Solid and well deformed finger rig.
10. PSD or better yet, a muscle rig.
11. Character UI: a solid one with mouse rectangle selection and shift selection that works bidirectional in viewport and UI.
Some other stuff I like to add:
Pinning elbows and knees
Pose library
GOOD distributed twist(god damn you Euler flipping)
space switching(more than just the head)
What Makes a Great Character Rig? 12 Things Animator-Friendly Rigs Should Have
http://blog.digitaltutors.com/makes-great-character-rig-12-things-animator-friendly-rigs/
No mention of platform?
a Pixar rig != a Halo rig
nice feature to have is a soft ik, so the joints dont snap when the ik is extended