I think I agree with a fair bit of what Ariosto is saying - namely, that pre-2nd ed versions of D&D presupposed quick and failry frequent generation of PCs, and that keeping parts of the game system in 3E (like save-or-die) while changing other parts (quick PC creation, hp totals, damage caps etc) can reduce the game's ability to provide satisfactory play to many players - both those looking for old-style play and those looking for story/PC-intensive play.
This is something I have been thinking a lot about as I run my PF game and create adventures (many of which might be described as "old school").
There are a lot of elements in D&D 3.x and PF that hark back to earlier editions, but don't necessarily quite fit because of other changes to the systems -- D&D-isms that don't really fit D&D anymore, if that makes sense.
Before I go too far, though, I want to acknowledge that 4E is actually better about this. While a lot of the terminology is the same, the actual system elements are not and much of 4E was apparently built from the ground up. It is one of the reasons some of us don't like 4E much (among others) but I want to give credit where credit is due: because of the "re-imagining D&D" nature of 4E design, it has fewer legacy mechanic issues.
Anyway, as an example let's look at the lowly dagger. In another thread, Bullgrit asked how often you AD&D and B/X wizard stabbed things with a dagger, either after his one or two spells were cast or in order to save them for the "big fight". Fire and forget magic users aside, one legacy mechanic issue this brings up is what 1d4 points of damage means between editions. In AD&D and B/X D&D, 1d4 was a significant amount of damage, and not just for a single level of play. Even creatures rolled hit points and those hit points were lower in general, so the possibility that a spell-spent mage whipping darts or daggers could still contribute to combat success was very real. Goblins were weak, and were "level appropriate" enemies for months or years of play. 2E started the initial hit point inflation without commensurate weapon damage increase, and 3E took that ball and ran hard with it. Suddenly the dagger wielding mage wasn't just desperate, he was a danger to himself and others and a liability to the party.
Similar things can be said about all weapons and characters engaging in melee of course, and by extension a lot of spells that cause damage. Magic Missile was a powerful spell, especially against "minions" (note that it did 1d6+1 in B/X and I think AD&D). Spiritual Hammer was a solid offensive cleric spell for much the same reason. Unfortunately, except for the big ass two handed weapons, weapon and spell damage remained static from edition to edition as hit points increased.
A similar example of legacy mechanics is the poor shield. In earlier versions of the game where a +1 to hit was a rare and valuable bonus, a +1 to AC was equally valuable. Moreover, the more abstract nature of combat -- prior to maneuvers and feats and the like -- meant that the shield's C bonus was "complete". That is, +1 AC effectively modeled a shield insofar as the combat system itself was concerned. But the combat system grew more complex and more granular as time went on, and the shield remained a simple, relatively small AC bonus, reducing its utility (especially as related to the previously noted two handed weapons). In addition, it's defensive values were not attributed throughout the combat system. (In PF, I let a proficient characters shield bonus increase his CMD and Reflex save versus area effect damage attacks; shields are the most commonly employed arm in the history of mankind, across all cultures for a reason.)
There are lots of other areas where tradition, legacy mechanics and "sacred cows" remain inherent in the game without taking into account all the associated systemic changes.
Further examples? Thoughts? Counter arguments?