They're not realistic, but let's face it - after we've spent ten hours slogging through wastelands filled with hundreds of monsters, we want something big to blow up. Or something fast. Or something tough. We want something which, once we finally defeat it, will allow us to shout "Yeah! Owned, bitch!" at the television. I mean, even in puzzle games, although the cry of victory may sound somewhat different.
The problem is that bosses are hard. Not to fight - but to design. Let's break bosses into a few categories.
We can get the easy one over with fast - the No-Boss. You wrestle through the entire game, fight your way to the Doomsday Machine, defeat the 817th group of faceless grunts, and the cutscene begins! You step towards the machine, a shadow falls across your face, and with a swoop . . . you turn the machine off and the credits roll. Wait what? Where's the climax? Sorry - you need something to happen at the end. Now, you can get away with the building/castle/planet/dimension falling down around you while you escape, or some nasty timed deal where you have to rescue the princess before she's eaten by zombies. But honestly, most people want a big fight at the end, or something they can use to demonstrate all the neato skills they've gotten dodging barrels for ten hours.
There's the Non Sequitur Boss. Sure, maybe you thought this was a first-person shooter. Maybe you bought it thinking that, like it said on the box, it was a platformer. Nope! The end boss shows up, your trusty sidekick hands you a spaceship key, and suddenly you're flying through space wrestling with controls you've never used before and never wanted to. Or maybe there's two bosses, both of which you must defeat simultaneously with half of your group available, switching between parties with a badly designed and kludgy interface that was clearly tacked on about three days before the game was released. This boss type is awful and has no redeeming features whatsoever. Seriously. If you want to frustrate the player, go for it. If you want a good game? Make the end boss at least tangentially related to the gameplay. Again, people want to demonstrate their skills! Let them!
Now that we've gotten the Crummy Bosses out of the way, I can go over the main categories of Good Boss I've seen.
First off, there's the Firepower Boss. He's like a normal bad guy. Only he's really, really tough. He's got big guns, lots of hit points, and is probably significantly larger than you are. Kill method: shoot him with everything you have, and then some. Avoid dying until he does. Problem solved. Firepower bosses are fun. The problem with firepower bosses is that we're kind of used to them. For a while, we had the problem that people would simply conserve their big ammo when they sensed they were getting near the end of the game. The final boss would show up and be hit in the face with all the most powerful bits of arsenal you'd found in the last five hours. (I think I killed the Quake 2 boss in under five seconds. Unreal's boss was famous because it could be killed in a single shot. Bad design, guys.) Luckily, we're solving this problem now. Modern first-person shooters don't let you carry a small army on your back and they avoid having one weapon far more powerful than any other - at least without major downsides to that weapon. So, the Firepower Boss is good. We enjoy it. But sometimes, we want a little more challenge than "do precisely the same things you've been doing for half the game." Hell, there are often standard creatures that can't be defeated solely with extensive firepower - why should the boss be universally weak?
On the other end of the scale is the Puzzle Boss. He can't he hurt with your weapons - and I mean at all. If you want to hurt the Puzzle Boss, you've got to figure out how to hurt him. Perhaps this will involve shooting four vulnerable points in a particular order (no, it doesn't matter what weapon you use, they'll all do the same amount of damage). Maybe you'll have to run from Point A to Point B rapidly while the boss attempts to annihilate you (get good at dodging!) and then jump through a giant hoop - repeat four times. Don't pick the wrong hoop. You'll figure it out. Or possibly the boss will cleverly have placed an enormous stone block directly above his head, suspended by a thin steel chain. Just get up there and shoot it with your blaster. We'll call it natural selection. Unfortunately, this boss has similar trouble as the Non Sequitur Boss. Let's face it, if we're playing through an entire game dedicated towards blowing things up, we probably want something at the end to blow up. On top of that, sometimes people aren't very good at puzzles. You either need to give them hints - in which case the puzzle is trivial - or you run the risk of your entire playerbase getting angry at you because, what the fuck, I shoot him and nothing happens! What now!
And then there's the Hybrid Boss. Yeah, you need to shoot him a lot to kill him - but generally, he's invincible until you do something clever. Hit him with an explosive so his shield goes away, for example. Dodge precisely so he gets hit by his own wall-mounted fusion cannons. And then, once he's reeling from that - that's when you pull out the portable nuke launcher and fire a few rounds down his gullet. Of course, this has some of the problems of both the previous bosses. People might not figure out the trick. People might not be great at fighting. People might be annoyed that there is a trick. Luckily, none of these are as crucial. You can give hints, because the puzzle isn't everything. You can tone down the combat a little because the combat isn't everything. Whatever part the player finds tough, they'll be content that they were clever enough to bypass it, and we'll just ignore the fact that the other one wasn't all that tough. Add some pyrotechnics and you're golden!
Well, not quite. There's one critical issue that the Hybrid Boss has. Being able to tell you're doing damage.
See, it's easy to tell you're damaging Firepower Boss because you're shooting him, and Firepower Boss has no defenses. Once you've shot him enough, he's done. Meanwhile, Puzzle Boss is similarly easy - if he's dead, you've solved the puzzle. If he's not dead, you still have to work on the puzzle. Meanwhile, Hybrid Boss is a lot more complicated. You might have solved the puzzle and just not shot him enough. He takes a lot of damage, see, but you've figured out how to do it, you just have to keep hammering on him until he dies. Don't give up. Or, maybe, you're doing fine on the shooting him part, but you're no longer actually doing damage. Perhaps the puzzle resets and it's time to solve it again. Perhaps you now have a new puzzle to solve. Or maybe you're just not shooting him in the right way - yes, you can eventually blow him away with your semiautomatic pistol, but you should probably take a look at the tactical plasma turret sitting on the wall behind you. Yeah, that one, over there. I mean, these things happen.
And this is a much tougher problem to solve.
Games where it's possible to "do no damage" usually have a way to tell which is the case. Arcadey games tend to have targets flash when you damage them. Non-arcadey games have the targets bleed. They both work. But you can't tell how much damage you're doing. If you're getting him down to 10% health that just means you need to be lucky to win - but if he's killing you while at 90%, it's time to find an entirely different strategy. And after ten or fifteen tries, if you don't know which is the case, you're going to be pretty damn frustrated. Of course, you could add a health bar, and some games do. But with others that would break immersion horribly. (I'm sorry, but soldiers in World War II didn't get a Hitler Health Bar on their heads-up display.) You can give hints, but hints kind of suck - if they show up too early they annoy people, if they show up too late they also annoy people, and everyone's "too early" vs "too late" is different. You can try to show damage in some graphical way, but it's difficult to manage something which is simultaneously visible, not irritating, and not game-effecting. (For examples of things which fail on different levels, you could show physical damage on the boss itself which they're simply not likely to see in the carnage, you could show little "oh god I'm hurting" cutscenes which will really piss the player off the fourth time he's forced to watch them, or you could make the boss start limping and lose weapons, which turns what could be a cinematic last-second win into a horrible positive feedback "okay I've clearly won but I still have to shoot him a lot to finish him off" situation.)
All of this is what gives rise to multi-phase bosses - it's easy to tell you're making progress when half the boss explodes every two minutes and you have new attack patterns to deal with. But those aren't suitable in "realistic" games, considering that we're already stretching the bounds of credibility by having a boss - no less a boss that is apparently designed to detonate repeatedly.
Super-Robo-Hitler will have to restrict himself to less realistic games. But don't worry - he'll be well-received there.