for me and pretty much everyone who plays online on the ladder. This is pretty much what the multiplayer is about is those 3 things, and I'm not really talking about the people who group to kill monsters because while that's obviously what the game is "about" at face value, you are severly missing out on the best parts of the game, which is building a character to be its best. There's a whole metagame beneath the game to touch. If it was just me I wouldn't say it, but there are a ton of communities built around it, the ladder got reset so that is equally true now, and well, basically the majority of people who play the game online really. And I'm not even counting the french, which get really really hardcore on it.
it's like trying to tell me the best part of StarCraft is not analyzing the maps and building unique strategies to counteract your opponent with micro/macro and knowing the exact metagame for each map with the exact way each unit from all 3 races interact with each unit on the competitive level like the Koreans and everyone who bothers to play competitively seriously does. It is the best part, and it is what StarCraft is known for, being the most balanced RTS to date. That isn't to say there isn't people who just play the single player, or just play with friends, but you are basically missing the best part of the game, and what the heart of the game is about. Yes, there is other stuff to do in StarCraft besides the highly competitive parts to it, and there is more to Diablo than the competitive, trading, min/maxing metagame, but there's also a single player mode to Street Fighter, and the similarity between all 3 is all 3 are something no one really takes seriously. It's fun to bash monsters in D2 with friends (I do it too), but there's more to it than that.
Once again, you're assuming being an obsessive hardcore gamer is the best way to play a game. You don't understand that I'm NOT missing out, that's what I'm saying. Just because you enjoy taking the game beyond into the metagame, doesn't mean everybody does or should. I can worry about min/maxing and ladders in other games that I'd rather play for the core gameplay. The sole reason I'm playing D2 right now is to crawl dungeons with my friends.
edit:
And your StarCraft comparison didn't really make sense as most of the things you described weren't so much metagame, so much as a staple of any RTS, and any of it that was RTS, is once again counteracted by the fact that I'd rather play the game instead of studying it.