Honestly, I really don't get this.
3e is far closer to 4e than to 2e. Right down to how you create characters. Heck, 3e makes the 4 character party the default, whereas previous editions generally pegged it at between 5-8 PC's. 3e presumes a 1 year campaign, thus rewards for play are all pegged at getting you from 1st to 20th in 1 year of play. 3e presumes a specific range of wealth and bases all design decisions ON that presumption. 3e presumes fungible magic items, prior to that, magic items didn't even have a PRICE in 2e.
You cannot even convert modules 1:1 from 2e to 3e (although, funnily enough, 2e to 4e works pretty well) as monsters are designed based on the 4 man party and not based on a single PC.
3e opens the monster manual up to players for character races.
You want to talk cosmology changes? 2e DIDN'T HAVE DEMONS OR DEVILS. Not at all in core and not for quite some time beyond that. People bitch about the changes to the cosmology 4e made, but, hey, this? This right here? How's that for all you Planar fans? Not a single lower planes denizen to be found.
The list of changes between 2e and 3e is massive. Every single facet of the game got changed. Compared to the changes between 3e and 4e? Not even close.