Passive buffs do get into dangerous territory based on balancing issues, but I am not opposed to them outright. I consider passive buffs for ridiculous leveling accomplishments to be comparable to the power increase seen in getting a Diablo 2 character from level 95 to level 99. There was a bit of a power boost there, albeit minor, so I consider a similar power boost for very difficult achievements at least in the same spirit, and hence tolerable.
Vanity rewards are completely acceptable, though they do not impress me. I expect most rewards will be of this form.
Tangible loot rewards (free drops, enhanced MF or Gold finding, free upgrades, etc.) I think are acceptable and will not interfere with balance at all, but I worry that direct item rewards would be as worthless as the item rewards in Diablo 2. Free items always sucked, and upgrade were almost never used until you had exhausted most of the lifepsan of the character and had nearly end-game items. I want to be excited about a reward and additionally want to use it immediately -- not save it for a few weeks.
A big worry:
Q u o t e: New random adventures become enabled
Something like this really needs to be a very easy to obtain reward. Building into the game quest content that is not seen by most people sounds like a waste of time and effort. If you wanted to activate a few extra adventures / mini-quests in a Difficulty upon completion, that might be ok. Pretty much everyone would get that achievement and hence the extra content, which I guess would be acceptable to me. However, I am still having trouble envisioning how this might be implemented such that it would be a satisfying reward. Perhaps the activated adventures could have an epilogue sort of feel to them (like what happened to some NPCs after the act / story was completed). Hmmm. Might you clarify as to what you thought would be an appropriate extra mini-quest?
Q u o t e: Or... we could just throw the baby and the bathwater into a volcano and then nuke the volcano.
Whoever created the Diablo spambots need to have their baby-makers taken from them.