Actually, there was an additional point I intended to include in my original post, but I forgot it until just now:
In D&D, there has been, IME, an underlying assumption that a particular party structure was necessary to be successful, esp. in a lot of the standard modules for D&D. This assumption more or less was the "classic 4" group of a fighter, cleric, mage, and thief (or some variation thereof). Provided just the right mix of abilities for success in a module (if well played & dice rolls were kind, that is...).
However, it's been a rather unspoken underlying assumption. There really wasn't a flat-out statement of "for best results, be sure to include A, B, C, & D in your group of PCs." IME, I've played in several games where this wasn't followed, and the PCs/players suffered for it, esp. in standard modules for D&D (severely in some cases).
4E doesn't leave it unspoken anymore. It pretty much states that the 4 roles (now broadened into Leader, Controller, Defender, and Striker) should be covered in a group of PCs. Each classes role (and secondary role, in some cases) has been stated in the class descriptions, so all participants are aware of each PCs part in the party as a whole.
I appreciate that as a player and a DM, because it really drives home the point that . Granted, a party without a certain role covered or only characters of 1-2 roles can succeed (in any edition), but sincerely, I'd argue it takes DM awareness & effort to allow that to happen. It's not going to happen often (or at all) if the DM just runs a pre-made/published module that works with the core assumption of the "classic 4" PC group going through it.
(Then again, I'd argue in previous editions, esp. 1E & 2E, there was an assumption that an elf or half-elf and a dwarf or gnome would be in the party, mainly for their abilities to detect secret doors/unusual stonework & traps, infravision, languages, etc., as well as their abilities to multiclass & thus cover more than 1 of the classic roles for the group. But I digress...)
Now, I'd say the reasons why you need each character role covered in 4E are different from why they needed to be covered in previous editions, due to the mechanics if each edition. But the need remains there, and IMO, at least 4E clearly states that it's there now.
This ties in to my original post & the comment there about 4e: unity. In this case, party unity: addressing the fact to the players that it's a "team game," where individual success is nice, but group success is vital.
I think that element had a tendency to get lost in some games, esp. due to players who wanted to/felt like they "needed" to have the "one character to trump them all" in that edition: whether it was a demihuman or a spellcaster in early-early editions; a psychic, monster PC, or Spellfire wielder in 2E; etc. The idea that a good character that could really bring something to the table for the party as a whole got lost in the process of creating a good character that works for itself (while it's ability to contribute to the group is questionable, at least).