Teamwork and Tactics baked in to the system
In 4e the team is stronger than the group as individuals. Defenders can do much more damage if they have allies. Leaders, especially Warlords, revolve around teamwork, and controllers are masters of setting people up for someone else to bash - but can rarely win a fight on their own. The combat portion of the game is one of teamwork; the only people who don’t directly both empower and rely on others are strikers. And the skill challenge rules when used narratively encourage teamwork in a way simple skill checks don’t - each member should be working out how to bring what they are best at to assist in the task.
In D&D Next, there seems to be precisely one ability made explicitely to assist your allies - the Guardian’s Shield Block. Also there is one spell in the preview (Battle Psalm) that buffs the whole party. Beyond that, literally every other ability a character has is ‘selfish’. Teamwork, especially focus fire, may happen. But you aren’t encouraged to play a group of people who can bring more out of each other than they would bring to the party themselves. The fighter does his thing (bashing) as the wizard does his. And so far there’s no group skill challenge mechanic to encourage players to work together that way.
I've seen this "required teamwork" touted as a plus by 4e fans for a couple of years and I confess to not understanding it. PCs who work as a team were always more successful in TSR D&D (and in non-charoped 3.x -- I can't say one way or the other about characters played by extreme charop players as I would never play with such a player) than those who did not do things as a team. Teamwork has paid off well since 0e D&D was published, even though it was not baked into the rules.
What I don't understand is what is actually gained by baking teamwork into the rules to the extent that 4e does. All I can see is that it limits the styles of play available to groups of players -- by forcing teamwork in the rules one eliminates styles of play that stress individual action. Without teamwork baked into the rules as to the extent 4e does, teamwork is still encouraged (simply because players who work as a team will normally be much more successful) but groups who aren't into that style of play aren't hampered by rules that almost require a specific type of teamwork.
I'd love to hear from 4e fans why they feel that "required by the rules teamwork" is so much better for D&D than simply having teamwork make parties more successful "organically" as teamwork did before 4e (and as it does in the normal world we live in). I see it as eliminating play style options that had been in D&D since the beginning -- and I never see that as a good thing.