There's tons to love about 4e. Some of my highlights:
1. Better mathematical scaling through levels
2. Ease of prep time, and game prep is FUN again!
3. The powers system is awesome and and very fun- no character is useless anymore or plays henchman to overpowered spellcasters.
4. Players are encouraged to work as a team and shine, rather than as individual stars. Party cohesion is tighter, which leads me to...
5. 4e is more roleplaying friendly. Yes, I've heard the screaming and ranting of taking out the profession and craft skills, but thats not what I'm taking about. What I have found in practice is that because tactics and teamwork are the focus of 4e rather than rules mastery, that the players tend to work better together as a team and come up with ideas as a group. That pulls people more into their roles as their characters and thinking as their characters, rather than distancing themselves from their roles and seeing their characters as simply numerical representations to milk every possible bonus out of their stats for optimal success. Its a subtle distinction to be sure, but its something I've noticed becoming more and more pronounced the more I've played and run 4e. And yes, I added back in profession and craft skills- I let every character take two trained skills not on the list in the PHB at character creation, but they are free to come up with some suitably narrow skill, such as cartography, carpentry, armorsmith, jeweler, etc.
6. Its harder to overtly powergame in 4e. System mastery is no longer paramount, instead cooperation, good ideas, and teamwork are.
7. Fast paced combat that feels dynamic and fluid.
8. Gear is less important than the character's core abilities. Something is seriously wrong when PCs fear losing their gear more than death.
9. I've always been a fan of minis. I love collecting them, painting them, and using them during a game. Integrating the minis into combat has been a big plus for me and my group. 4e has the most clear and concise rules of any D&D game to date, and it plays sooooo smoothly. I found my attentional resources have been freed up greatly not having to worry about monster feats, touch AC vs flat-footed AC vs normal AC, etc.
10. Rituals are awesome. I love the flavor of them and that they can be available to anybody with the right training. I anticipate seeing a lot of the "missing" spells from the previous versions of the game showing up as rituals. I also like the increased time requirements of rituals- it requires some player forethought, and I've already seen some very creative uses of rituals. I always felt magic in previous versions of D&D was too easy and quick, and rituals make magic feel more "magical". Plus, rituals allow me to simualte movies, books, and legends better than previous versions of D&D did- how many times have we seen someone untrained in magic bumble a ritual and cause all kinds of havoc? (klatu-verata-nik...coughcoughcough).
11. Last but not least, 4e feels more like D&D to me and my group than anything has in a long time. 3e never felt like D&D to me- the rules were too much in the foreground and overly complicated and interconnected, and it felt limiting or constraining to us, despite (or maybe because of) all the options the rules present and the assumed magic levels and mathematical model. True, 4e mechanics are different and streamlined for AD&D 1e, but the feel of the game is more like 1e- and thats an awesome thing!!! I can't exactly explain why though. Maybe its because monsters are new and interesting again, maybe its the more rules-transparent nature of 4e, maybe its the lower assumed magic level, or maybe all of these, as well as something I haven't put my finger on. I can see design elements the designers were trying to replicate from 1e to get that "old school" feel back, while still keeping the game modern and easy to understand.