My comment was influenced by the fact that I played with two guys who were optimisers. They had their feat selections planned out through level 20 and were brutally effective. That left the rest of us with the choice of matching them or practically sitting out half the fights. My character concepts were totally useless in the face of that. I had to rebuild three times in 8 levels just to participate (and I'm glad my DM let me).
Which comes back to what we all play D&D "for". Some people get a kick out of tweaking the system (just like hot rodding a car they can't legally drive or over-clocking a PC used for web surfing to 5 Ghz), and 3E was great for them, but their presence made it a lot less fun for people who were there for the roleplaying/storytelling aspects of the game. It was like showing up a friendly game of pick-up basketball only to find that one of the guys had modded his sneakers to make him run 2x as fast and jump 2x as high. It's a neat trick and cool proof of concept, but it doesn't make for good basketball.
Those guys were optimizers, and I understand what you're saying about this aspect of some gamers. I don't find this fun either.
But what I think most people are saying on here isn't that they enjoyed the intricacies and flexibility of 3E because they could "optimize" characters for maximum lethality, but that they enjoyed the intricacies and flexibility of 3E because of the way it had a mechanic to support just about any roleplaying concept, and therefore maximized the roleplaying experience for them. There are just too many concepts that can't be envisioned in 4E (and "keep the mechanics, change the fluff" is not the answer for these players - including me).
There are lots of different reasons that people play RPG's. Different people get different things out of them. Just like that test that determines what
type of gamer you are (narrative, simulationist, gamist, etc). There's nothing wrong with any style of game, other than when conflicting styles are mixed into the same group. But that requires an entirely different kind of resolution. However, the problem of conflicting play styles will never be fixed simply by making changes to a game system.
Just because the system could be used to make "optimized" characters that, although legal with RAW, broke the spirit of the RAW, doesn't mean there's a problem with the system (it's not necessarily a problem with the players either, as long as the group you are playing with prefers or doesn't have a problem with this type of play). In any codified structure, there will always be loopholes. Even though they haven't been found yet, it doesn't mean 4E doesn't have them. There will be powergamers in 4E just as there has been in every D&D edition and every game system. There will be "optimized" characters that "break" the spirit of the systems rules, and will make the game "unfun" for some. The fact that this is possible in a system does not make the system bad.
I don't Feel that 4E is a bad system or game. It very definitely is a different game than 3E, but no more so than 3E was a very different game from 2E. 4E achieved what the designers set out to accomplish, it is a balanced, fast, simple, and fun system to use. Playing it will provide just as much enjoyment to those who like and use the system, as any other game or system has provided to those who like and use those systems.
I just don't understand why the "concepts" that were thought up as fixes to percieved problems in 3E, couldn't have been applied to 3E, rather than completely stripping the system down to it's bare bones chassis and then rebuilding it into a very different game. (That is, I can't understand why they took this course, other than they needed to make a "new" game in order to keep making money - and there's nothing wrong with that. However, this means that eventually, there will be a 5E. If for no other reason than they will need a new system to rejuvenate income. Though, there's no way of knowing how long that will take.)
Anyways, back to the subject, I think that's why most answers to "What is missing?" seems to be
Complexity,
Variety, and
Flexibility.