How is unfair to the other players if one player comes up with an encounter-ending idea? Can't the other ones do so as well?
Sometimes they can, sometimes they can't. Different players have different levels of "creativity". Some of the people I've played with never come up with ideas that are very outside the box. They don't think that way. One in particular comes to mind. Some players feel like I do, that such things are "unfair" and they've rather have a fair fight with the enemy. Either from a sense of personal honor or roleplaying their character's honor. Other times, it's a matter of resources. The Fighter has a sword. The Wizard has 20 spells, each of which when interpreted broadly enough can have way more effect than the game designers likely intended. The Fighter likely has an 8 Int and when roleplayed correctly doesn't come up with super intelligent ideas. The Wizard has an 18 Int and should be expected to come up with these things. This adds up to the Wizard doing crazy out of the box ideas that defeat the enemy in one round nearly every combat while the Fighter is restricted to "I attack it with my sword."
For me, it's rarely the knockdown drag out fights of attrition that I remember. It's the flashy ones in which unusual and different things happen that stand out in the long run. It's one reason I think 4e may not be doing so well. You fight lots of grindy fights, and what stands out years later?
I don't know, I remember the hard fights. Like when we fought a primordial. Or that fight where the giants kept kicking my character into the freezing water. I'd run out and they've kick me back in again. Or that battle where all the enemies drained healing surges and we had to pull out our big guns to kill them as quickly as possible.
Yes, in each of those cases it was something "unusual" happening. But they were unusual and still within the confines of the hitpoint system(except maybe the healing surge loss, but that's a topic for another day). The lake did damage when he kicked me into it, but it wasn't more damage than he could do with his normal attacks.
That suggests, to me, that you put too much of your own ego into your creations as a GM. You can't really choose what elements of your game the players will remember and think well about in years to come. You can't expect your players to not mess up the encounters you set up without railroading them away from doing so. If faced with a similar issue, I would simply tell the players that they outsmarted the BBEG, who are always built with the recognition that they are one guy who can't possibly account for everything the PCs can do, and then graciously congratulate them for having a cunning plan. As I see it, that's part of DM's job. It's like being the Washington Generals to the Harlem Globetrotters.
Yet, years later when they tell the story, it's still to make fun of me for not planning ahead. Not pride at how they defeated the enemy. Even they will admit that the reason they keep telling the story is because it was so funny. And it was funny because it was the exact opposite of expectations. They expected to nearly die...instead they didn't take any damage at all.
I've seen them fight long, drawn out, difficult encounters. They take pride in defeating them and enjoy them. Which is why I try to make sure at least the boss fights are like that. But they also enjoy killing things in one hit. Killing things in one hit is no fun for ME however, so I rule that it doesn't work(at least in important fights).
As for ego, I think some DMs are TOO neutral and selfless. The goal of the DM is to make sure EVERYONE, including the DM is having fun. If the players have fun in a way that makes the game no fun for the DM, then it needs to be changed. I refuse to be a martyr just to be the DM. It already takes a lot of work, if anything I have a little MORE right to have fun than the players.
Also, it isn't about ego. It's about spending 3 hours writing up the stats of an enemy, 6 months of playing time building suspense and tension to the final meeting of the ultimate boss only to have him die under a pile of rocks while he's in the middle of a monologue about how the PCs are doomed. It's a waste of time and effort for me.