Once the game goes live I could see the potential for people to get bored once they have acquired all the skills and no longer have the ability to improve their character. Perhaps having a skill decay that goes into effect if you don't use that particular skills for a certain period of time. If you were to stop playing for a while then came back you would need to re-learn some skills (perhaps at a quicker rate due to the "muscle memory"). This way you are somewhat forced to be a more balanced player and do many things as opposed to just doing the same things all the time.
Maybe activities could prevent skill decay in all skills but have a bigger effect on the skills they specifically relate to. For example, mining everyday would mean your mining skills would not decay AND it would also slow down the decay of all other types of skills (although specific skill related activities would still need to be done in order to fully prevent/reverse the decay in the non-mining skills).
Maybe this has already been discussed. Maybe there is a better solution out there, other than an infinite skill tree.