Lots of good thoughts around here.
I personally go with Hassat Hunter here. I would myself like to not have any boss at all. There are other ways to do difficulty peak.
On the other hand, I'm also with Dr Koin saying that bosses are pretty often the end of a part/zone/chapter. Which is great. You know as a player, when killing a boss that you achieved something in the game, progression, or something like this.
The bosses I like in game usually is, like Lacrymas said, the ones who test the stuff you learned in the zone.
For me that was not the case in D:OS, it was mostly big bad dude/dudette who where here to punch you in the face, sometimes cool, sometimes cheap. It's ok to some extend with the game because there's no way to exactly know how the player will spec their characters. In D:OS you can know, to some extend, which skill the player might have but that's still a lot variables.
It's not like Super Mario Bros or The Legend of Zelda where this is the most simple form of Rational Game Design. In this particular level you give this power to the player, he learn all the way through the level how to use it and then the boss is there to challenge him.
Designing boss is a great challenge. Especially designing FUN bosses. Because designing nasty annoying boss is pretty easy. But in the end, they are not just fun. We could go to Darksoul on that matter, bosses destroying you, that are fun. I guess that's another subject, haha.
In MMOs and a few others games, what's interesting is when encounters start involving elements of decor in the actual strategy of the boss. Like forcing them to bullcharge in the wall, attacking totems in the right order, having a wall or the ceiling or the floor collapse unto/under him, forcing him into an electrified pool, etc. A multifloor fight may even be possible in DoS despite the top view, just fall with the boss and maybe suffer some light crushing damage.
In God of War 2, we fight two of the three sisters of fate : the fight essentially ends when we trap them into the mirror around the room by smashing them. They use those mirror to send us back in time where we have to ensure what happened then still happens.
The second kind of tactics not revolving around abilities or gear is targeting : you have to destruct parts of the boss ( whose appearance can change, ie armor crumbling, new parts emerging, etc ), or adds first/in order, or ( and this one is a dungeon boss from WoW, Jandice Barov, that they redesign some time ago ) attacking the right clone. In the WoW example, she splits into like a dozen or more clones that are clothed slightly differently from the original - that's how you can find her. Of course you can brute force and kill everyone.
Another example involving both room mechanics and targeting mechanics is Yogg Saron ( and C'thun too ) from WoW - simply put they "ate" the players who are then sent to another room where they have to fight to get out AND hurt the boss. The Lich King encounter also sucks players into his sword, and they have to fight the trapped souls in there to break free.
With 4 characters available, multi-planes battles may be a reality
Of course these are very basic examples. Games with the best boss fights I can remember are God of War(s), Darksiders wasn't bad either, Castlevania:Lords of Shadow/Shadow of the colossus ( since well, it's all about climbing giant monsters in both )... I'd tend to think it's harder for cRPG bosses to leave an impression as they often lack the theatricals from TPS or modern Beat'em'all, but there may be a little revolution at hand here...
