4) Controllers. What is a controller? What do they do for the party? Is it AoE? No because even fighters and rangers have lots of AoE. Do they mess up monsters? Not as well as dirsuptive strike or confounding arrows does. In past editions a wizard was needed because he had area attacks and could do cool and interesting things that other classes couldn't. Now every class can incapacitate enemies with status effects, and has tons of close or area attacks, making the wizard's-role (redimagined as the controller) unnecessary.
I have to disagree with this one. A melee druid isn't a great controller, and they kind of goofed with the PH1 wizard, but they're starting to figure out the controller's role. It does AoE and status effects. Other classes can do this to an extent, of course, but the controller is the mighty grandmaster of it. A wizard's crowd control is like a cleric's healing. Sure, you can Second Wind, but ain't nobody going to hold a candle to the cleric.
Also, as a DM, I find it's often the controller that foils my sinister plots. This actually happened in one of my games: "A team of gnoll archers atop a high battlement open fire on you! In front of them is a ten-foot ditch filled with ravenous hyenas! How will you ever reach them in time?" The invoker wins initiative. "Great," he says, "I cast this spell, and drag the entire team of archers off the wall, letting them fall 30' into the pit of hyenas." And he did. Basically insta-killed what I was expecting to be a very difficult encounter.
That's controllers for you. They appear to do nothing for a long time, and then they pull out the perfect power at the perfect moment to utterly wreck your face. The standard pattern seems to be: crappy at-wills, game-changing dailies.
I'll admit, they're not exactly necessary to a successful party the way a leader is. But they certainly help round things out.