- Well, one thing is a visible healthbar, I hate when in games there's a huge boss monster and I can't see if my attacks are actually doing anything. I don't remember any specific games but I remember that in some games I've wasted all my ammo into a monster (or whatever) and though "hmm, so obviously this is not how to defeat the boss..", I try again looking for some other way of defeating the boss, but don't find anything and I finally look it up on the internet where I see that you're simply supposed to shoot it until it dies. This one is almost the most important one for me...
- I want the battle to last for some time, hopefully to some epic battlemusic. It shouldn't take too long though, enough to build up your victory, but before you get sick of the boss.
- Make it somewhat dynamic, I hate bosses which makes you memorize some specific pattern before they can be defeated, it's not a test of skill when you encounter that boss one time and he pulls some random shit out his ass. Not that a boss with patterns is bad, I'm just saying that the battle shouldn't be ABOUT the patterns.
- It must be a good boss, the character, the battle may be well made but unless the boss himself is either awesome or built up throughout the game you won't have a good time. Deus Ex HR, great game, bad bosses, the fights were very badly designed but IMO the worst thing about them was that you knew nothing about them, you get more story from them from trailers than you do in the actual game. They might as well be an extra tough guard for all I care, they're quite bland and uninteresting, making the bossbattle so much worse.
Games I think has gotten this right, the MGS games and Gta4 (and RDR). The MGS games have always had fun boss fights and they're always presented in smart ways, overall they all feel epic and most importantly, they test your skill. I also said Gta4, strange huh? That game got no traditional boss battle, no, but it does have bosses! And I like the few there are since the "boss" is well built up, the battle is leanghty and they have a special feeling, winning gives you satisfaction.