I like both games, so I'll take a shot at it:
PATHFINDER/ 3.5 TRAITS
LOTS of character customization, particularly if you also use 3.5 classes/ prestige classes.
Familiarity: We've played this game for 10 years; we know a fireball does (level) d6 of damage like we know our own street address.
You can have lots of followers/ pets/ undead servants, and each one gets its own turn.
Spells and effects are interpreted based on their descriptions; if sneak attack represents hitting a vital spot, then things without vital spots can't be sneak attacked.
Save-or-die and Save-or-suck effects are long-lasting and feared; level drain is worse than dying.
Non-casters increase power arithmetically; casters increase power geometrically.
Monsters and PCs are constructed using the same rules, making the preparation of complex NPCs just as time consuming as building a character, and making simple changes like "-2 to Dex" very fiddly to implement in mid-encounter.
4.0 TRAITS
Some classes have very little customization possible; two players with vampires are going to look very similar. This used to be a problem with every class, but not so much after a couple of years of material.
Generic-ness of powers: There are very few "iconic" powers with the memorability of
Charm Person or Stinking Cloud. Hearing a power's name ("I use Vicious Smite!"), even after years of play, doesn't call to mind its effects or even what class uses it.
Clearer powers: Remember all the fights about Charm Person? Gone. Now it's just "slide it 3 squares and it attacks its ally".
Balance is king: at every level, casters and non-casters are of comparable power. It's not perfect, but it's an order of magnitude closer. Detractors will phrase this as "wizards got nerfed" or "everyone is magic now".
Save-or-dies almost gone: You generally have to fail 3 saves to die, giving your party plenty of time to save you. Debilitating effects like petrify last about 2 rounds on average. No level drains, and even rust monsters are a minor inconvenience. Nothing is allowed to permanently ruin your character.
Spells and effects are interpreted based on their mechanics; if your attack "knocks prone", and prone is defined as "grants combat advantage in melee, but gains +2 to defenses vs ranged attacks, and requires a move action to stand up", then that is what happens, never mind that the target is an ooze, a fish in the water, a swarm of army ants or a snake. It's up to you and/or the GM to come up with a description of how you achieved that condition on that target.
Conditions are still too numerous and needlessly fiddly, but easier to apply in this edition, since they tend to be direct subtraction from a combat stat, rather than from a primary stat that has other stats derived from it.
And the biggest reason I GM 4th now and not PF:
DDI. Character builder is awesome, Encounter builder is da bomb, and Compendium online is hugely helpful. I can prep an entire session of 4E faster than I can apply the Lycanthrope template to a bugbear in PF.