Damn, I was hoping this thread would be SHARK's analysis of the two editions.
(I can wait.)
Me too.
In answer to the question from someone who likes both editions...
Fourth edition does two major things in its revision; streamline and balance play and create an all-encompassing "world". Both have met varying degrees of flack.
To the first, the major revision has been the powers system replacing spellcasting, 1/2 of the feat engine, and class abilities by rolling them into one all-purpose "power" structure. All classes have a predetermined amount of powers at a given level the constitute attacks, spells, combat maneuvers, class abilities, bard songs, rages, etc. This creates a game that is inherently more balanced between classes, with the side-effect of a level of "sameness" with powers (as in, most powers have an ability score vs defense to hit, deal damage and grant an effect, and are mostly differentiated by fluff and keywords)
Beyond this, skills have been condensed into a a dozen or so "adventure useful" skills (no profession: cartwright here!), saves have been turned into static "defense" scores, hp has been increased at the same time damage has been toned down (esp in spellcasting) creating longer, less-swingy fights, movement is more important (bring with it a battle-mat friendliness), magical gear is less important (though still useful) and healing has been radically changed to fix problems with healing and clerics (as well as give all classes some self-healing).
At the same time, D&D's "fluff" has been dramatically altered. While elves, dwaves, giants, vampires and dragons are all still present, each received a "re-imaging" to update the mythos. In addition, newer races that appeared in later editions (like 2e's tieflings or 3e's dragonborn) became first-round race choices; many outsiders were removed or changed, some monsters got higher or lower "levels" to accommodate a 30 level game, the great wheel was tossed for a simpler cosmology, the affectionately called "points of light" world became the non-setting default setting, Forgotten Realms and Eberron have received updated to comply with the changes in the core world, and some famous faces (druids, gnomes, metallic dragons, frost giants) were held back until the second round of core books (coming in 2009) come out, expanding the concept of core beyond "SRD-only" to "All books with PHB/DMG/MM on the cover".
Beyond that, all you've missed is the sad state of GSL (4e's OGL), the death and rebirth of D&D Minis, and a lot of shouting matches that involve the words "grognard, video game, WoW, anime, Mini's Game, MMORPG, CCG, and not-really-D&D"
Welcome back!