No jump button please! Why? I think i let this video explain it for me.
http://www.youtube.com/watch?v=ICK7rDO6WtY

A terrain triggered jump like in FFXIII would be totally ok thou or climbing up like in Monster hunter.
Jumping isnt the problem in itself, its the people who abuse it, and boy they WILL abuse it if its implemented, as they do in ANY MMO that has a jump button.