Well, there are two things that immediately grab me and keep me interested in 4E.
1: Melee and magic are basically equal.
2: Characters are mostly human in abilities and movement.
1: I really disliked how casters dominated games in 3.x, or swung from useless to gods in only 15 or so levels. Too often melee waited for casters to peruse and discuss spells, or asked them to cast specific spells to help them do their jobs, be they buffs, dispels, or just movement powers to save them time.
2: Teleport, or scry-buff-teleport, flight, invisibility, levitation, etc etc etc made building encounters hard, and even harder as he characters hit about 7th level or so. Sure, the DM could nerf the players, or the players could avoid strategies like these, but why did they exist at all? Maybe 4E went a bit far with knocking these off the table, but I like them in shards on the floor.
---------
Other things I love about the game:
Minions--Pure genius.
BLB--Help high level fighters not be totally blind, and help with scaling.
Skills: Fewer and less difference between those who were good and those who were not. I remember many 20 point differeces in 3.x.
Skills: Everyone is better than the average joe schmo on the street after level 10 or so. When your 15th level fighter had a worse spot than a commoner with a 12 wisdom, something was wrong.
Prep--Man, is it easy. With roles, simple xp amounts and a good selection of levels and roles next to each other in the MMs, it is a breeze.
The Classes: I really love how WOTC has done the classes. Same role, but totally different feel. For example strikers. Avenger rolls two dice, the Barbarian hits hard with a weapon, Sorcerers have crazy base damage so that the die roll is not even needed, the rogue weaves in and out of combat, the Ranger can shoot a bow or attack with double weapons with ease, and so on. All the same role, all effective, all different in how they do the role.
Non-divine Healing: No need for a cleric. Try a bard, a warlord, an artificer. Again, all the same role, but very different in how they pull it off.
--------------
Now let me talk about DDI as it was specifically asked about.
Mainly I put that line in my sig as I was getting annoyed by people constantly suggesting I try DDI, as if I had never heard of it. Well, I ahve heard of it, and it is probably big for many or even most people, but I see no need for it.
Character Builder: How many characters do DDIites make anyway? I am sure it is easier to make one than on scratch paper like I do, but I only make a new character once a month or so. How many games do you people play in anyway?
Monster Builder: In a year and a half, I have used perhaps 30-35 monsters total in 3 PBEM 4E games. I have the first two MM books and have only used two from the MMII. With reskinning and modifying abilities slightly I have thousands of potential monsters just from the two books.
Encounter Builder: No real opinion on this, but I like a white sheet of paper to plan things on.
Three other things and then I will stop this.
1: Everything is Core. Terrible idea, but with DDI it is a lot easier to do so. Yes, it will help well books, but it is BAD for the DM. I do not like kitchen sink campaign worlds. And parties with 4 of 5 unique races/classes/ etc is just unbelievable. All wizards died thousands of years ago except for him, and all elves migrated to the stars except her, and this guy is the first artificer the world has every known. .......
2: 3P, House Rules, and Campaign specific stuff. If you have stuff on paper, this is all easy, if you are married tothe character builder and its cousins, not so easy.
3: And a pet peeve of mine for the Character builder and all its antecedents over the years. Yes, you can make a character with this, but do you know the rules behind all those numbers well enough to play it effectively?
I'm not trying to bash anything here ,and I am glad lots of people like DDI, though mainly for a selfish reason. The revenue stream will keep WOTC around to make the books I want to buy. The nine books I ahve bought for 4E thus far have probably netted WOTC less than 50.00 on their balance sheets. That is not going to do a lot to keep WOTC alfoat. The 10.00ish a month (of which WOTC probably nets 80%+) is better for that.