|Everything you mentioned, but besides the anti-reverse, the B's have a more balanced spool so you don't feel wobble when you turn the handle, the older one's you can feel the wobble. Forgot what this feature is called, but common on most spinning reels now. The B's are also larger than the older models, the 3500, 4500, 6500, seem much bigger than the older counterparts, both in the body and spool. The older series baitrunner spools are not interchangeable with the B series. That's about all I can think of, I'm sure there's more internal differences.
Let em go and let em grow