Healing Surges were by far my favorite introduction in 4e. It brought a lot of good things to the game:
-Allowed everyone to have some moderate control over self healing. (Second Wind, I love it)
-Made it possible to make people eventually run dry and need to rest without relying on vancian spellcasting, and prevents items like potions and scrolls from making you last forever and a day.
-Gives a resource you can penalize for things like diseases, or harsh environments (see: Dark Sun) before resorting to straight up ability damage.
-Gives defensive focused characters a stronger advantage than just base HP. When squishy over there has a healing surge value of 8, and 6 surges, while Tanky McTank over here has a healing surge value of 14 and 12+ surges, the party is rewarded for him doing his job right.
If my text is blue, I'm being sarcastic. No, really. Don't bite my head off for saying the Fighter is way overpowered, or how Wizards need more spells per day to keep up.
I like skill challenges. I can see now why they get a lot of criticism, but I thought the idea was fantastic when I first read it.
Also, it's more teamwork oriented. 3.5 was like naval combat in the Pacific theatre of WWII: Everyone form a barricade around the aircraft carriers (Tier 1 and 2 spellcasters), and then they'll win the battle for us!
In 4e, it's more like...well it's more like a sports game. Yes, it's possible to have a star player, but he has to earn that spot, and it will change based on the type of opponent you're up against.
Lotsa minions? Time for the Wizard to shine! Big boss? Ranger, you're up to bat! Big boss that no one can hit, auto-damage from the Fighter and Paladin wear it down overtime. Still challenging, but you will ALWAYS be important in some way or another.
1. Everyone is important! I remember playing a 3.5 game where I played an insect-themed druid and had a giant stag beetle. Our poor Paladin was all "That dumb bug does more damage than me!" I know people have varied opinions on balance, but I really do think the game is more fun when everyone is around the same power level.
2. Vastly improved monster design philosophy. I understand where they were going in 3.5, by making monster and PC design unified and having an internally consistent logic, but I really don't think it ultimately made the game any better. I remember making my own monsters and improving existing ones and it was a real pain in the butt. I enjoy 4e's approach of just doing what you want for a monster's powers, referring to a chart for attack bonus, defenses, HP and Damage and you're ready to go! Even then, there are enough monsters in the Monster Manuals that you can easily take an existing one, change a few details and adapt it for what you want.
3. Exciting combat! Not to say 3.5's combat was bad or anything, but I really, really like how 4e combat flows. I like how battles have a little length to them, to make fights really feel like a back-and-forth struggle, rather than just two sides clashing and one getting ripped apart quickly. (This may have something to do with a 3.5 game I DM'd where this would happen almost every battle. A Feral and Maniac-templated monk may have had something to do with it... >_> [That's all probably my fault, too, for allowing it, but that's a whole story of its own.]) But yeah, I like how the save-or-die type stuff isn't there. I prefer combats to feel like a BATTLE happened, not just a slaughter.
4. No overpowered combos! Okay, so this isn't 100% true (there are a few, but they involve parties full of Warlords or ridiculous Ranger builds) but I know that in 3.5, overpowered builds could be found around every corner, with a little searching. In the same campaign as stated before, I spent about an hour figuring out how to advance this boss monster (The Ulgrastasta, if anyone cares to know) to make it actually stand a chance. What happens? It gets nuked in one hit by a ludicrous Arcane Thesis/Eyeray spell combo (at least 500 damage off of that). Could I just disallow this sort of stuff as a DM? Yeah, sure, but nobody likes to get told "no." For an optimizer, to get one of your best options axed sucks. I'd rather such things just not be possible to begin with. Then, the ones who didn't go searching for the most broken combos are left in the dust. (I feel sorry for that poor, poor Spellthief...) Really, I tend to think the super-powered builds tend to be more fun to make then to actually use in a game.
Yeah, I know, I think I came across as pretty harsh against 3.5, but I don't hate the system at all- my best games have been in 3.5. I've played more 3.5 than I have 4e and my current game is 3.5. I just really like 4e a lot more (and hopefully I'll get the chance to play more.)
While I don't personally enjoy 4e as much as previous eds (I fear change! :O ) I'm happy that it's introduced a fresh influx of new gamers to the hobby. More people to play with is always awesome, regardless of the system!
The encounter system in particular, in my opinion, was much improved. Just the way you set up monsters is infinitely more intuitive than the CR system, but just the ease of DMing in general has made things more fun for me. I can spend as little or as much time as I like on a session and I don't really have to worry.