This is more about RPGs in general... but they could all be directly applied to
UO and many of the current major discussions that have been occuring lately.
Anyway... here's a few lists of ways I think things can be changed that are easy to balance and/or have such a small effect on balance that nobody would ever really complain about them:
Crafting Durability - how long something will last. Large bonuses to durability could be an issue, but anything up to a 100% durability increase should be easily acceptable
Weight & Size - limiting factors on how much you can carry. Again, any small adjustments should be acceptable.
Stat/Skill Requirements - This can be partly balanced by making it less effective at the lower levels than at the normal minimum requirement levels.
Combat Adjustments Damage & Speed - Increase one, decrease the other, but maintain the same DoT. As long as healing ability is balanced (which it isn't in
UO) so that neither doing high damage per hit or hitting really fast will be a clearly better choice, it should be fine.
Chance to Hit/Be Hit - Increase one, decrease the other.
Healing Speed v Ability to take damage - Increase one, decrease the other, but maintain average DoT.
Taking damage of type X v damage of type y - Increase the base of one kind of resistance, decrease the base of the other kind. Naturally, this should never allow anybody to become invincible or nearly invincible to any type of damage - but as long as the best resistance is still reasonable (personally... I think 70% is too much), it should be fine.
Thats everything. Pretty much everything is sucky when it comes to ease of balance