I want to see bosses that require actual concentration to beat. Bosses where simple tank & spank or zerging aren't enough. Bosses that cycle weaknesses, bosses that require certain conditions at certain points in the fight, bosses that aren't easily predictable.

A good example of this are the bosses in LotRO. Sure, some of them can be killed easily. But there are plenty that are challenging, strategic, AND entertaining ("Would you like that pan-seared!?").