I have been a dungeon master for years and haven't played all that much since the early days of 2E, but here is my list for 4E:
1) The ease of monster creation - make what I envision not what the numbers dictate. I love making and modifying monsters on my own, and the new rules make it sooo much more fun and easy.
2) The addition of quest experience to allow a story driven campaign rather than entirely monster killing campaign. I have grown so unhappy with the CR experience system that I ditched experience in general a long time ago, and I tell the players when their characters level.
3) The removal of spell-like abilities which are usable 1/day. These always sucked and never allowed original MM monsters to incorporate later books spell lists.
4) The simplification of the skill system that utilizes only the skills that are necessary to a party of heroic adventurers. I never cared about farming or professions, and I hated that stealth was 2 skills. The new system is so much more intuitive and useful to my designs.
5) Minions. I love minions. I will have my group fight oodles of minions because I love it. Love it - and so will they.
6) Spontaneous actions are the bomb. I love that actions on the battlefield actually seem to effect everyone. Currently most characters are on an island until their next turn. Now they will always be focused and paying attention to the tactics of the combat to utilize those unique times when they get an advantage.
7) Combat Advantage - Simplifying all the silly effects into a single catch-all term and effect is brilliant but not dumbing it all down. I hate looking at the back of the DMG for the exact effects of Frightened or Dazed for the 100th time.
8) The heartiness of low level characters. I hate having lame encounters for levels 1-3 that don't seem interesting or challenging simply because I don't want half my group to die and reroll after spending 6 hours discussing who is gonna play what.
9) Less die rolling. My group is high level, and it can take _forever_ for an entire round to complete. My warriors get four attacks, possibly having to confirm criticals hits, calculating extra damage from rages, smites, special items, is the weapon two-handed, am I power attacking....ugh...too much time involved in making 4 attacks all of different attack bonuses.
10) The defenses. I love that everything attacks defenses rather than having my group roll their saves. I think this is the best modification to the core DD ruleset that has been introduced since the removal of Thac0.