I'd probably be considered "old guard", having gamed for 25 years, and started with AD&D 1e.
And for the record, I LOVE 4e. I'm an admitted 4on, and proud of it! IMO, it is close to what the game needed to evolve into to stay viable and fresh. I don't think WotC ignored the old guard- it listened to what players from the previous edition wanted and their criticisms, and tried their best to fix many problems. Its not perfect, but its a huge breath of fresh air over the old and stale D&D tropes of the last 30 years, but it keeps enough to make it recognizable as D&D.
At the end of D&D 3.x, the fanbase was already badly fractured, and the system was in dire need of overhaul. As has previously been established, 3e broke new ground and introduced a lot of cool innovations, but the math of the system broke down, casters were horrendously overpowered while non-casters were baggage, and the system relied too heavily on magic items. In addition, the prep time for running a 3.x game could be horrendous if you went by the books, and IMO I felt like I was always fighting the system to run the game. Apparently a LOT of other folks had these same complaints, because the main design goals of 4e addressed these problems, and did so in a mostly successful fashion IMO.
I fall strongly in the simuationist camp of gaming- I want the game to be able to handle a logical and coherent world. To a large extent, this is governed by how the DM presents the world to his players, but system can play a pretty big part too. 4e has some problems in this regard, namely in healing surges, and the at-will/encounter/daily power divide. But even with these problems, I've found 4e a MUCH better game for my gaming style than 3e. Hell, we've even got house rules for longer-term healing, permanent injury, and a modified way of using powers that makes us happy. My group and I gave 3e the old college try, and after 4 years of frustration (and thousands of dollars spent on 3e books) of not being able to have the kind of game we wanted, we gave up 3e. We've also done something with 4e we'd NEVER been able to do with 3e- get non-gamers (and female ones at that!) into the game, and they have responded very positively towards it! Now that is exciting! We're having the best time we've ever had playing D&D with 4e.
So my list of likes and dislikes for 4e are:
Likes:
* Standardization of improvement for BAB, skills, and a SOLID mathematical model for the game
* Class roles, and the flexibility of powers in customizing a character that truly plays and feels different than others of the same class
* New multiclassing rules
* Reduction in power of casters, and increasing power of non-casters (although in practice, the 4e wizard is still extremely powerful when played well)
* Trained/non-trained skills rather than dinking with skill points
* Combats are fluid and dynamic by design, and tend to be more chaotic and exciting. Happily, the days of toe-to-toe full attacking with no movement are dead and gone.
* The same assumptions are not used for PCs and monsters, any why should they be? Monsters have screen time of a few rounds most of the time, so why bother with niggling enormous stat blocks? Monsters also play very differently than in previous verisons of D&D, and differently than each other, which is a great change.
* Moved away from Great Wheel, stale cosmology. The Shadowfell, Feywild, Astral Sea, and Elemental Chaos are far more evocative to me.
* Rituals are awesome- the best thing to happen to spellcasting in a long time.
* A powerful build now won't break the game, and builds are no longer the focus of the game. 4e is more about playing the game and using teamwork. HUGE improvment, and I've seen guys obsessed with builds in 3e turn over a new leaf and actually roleplay and work with the rest of the group!
Dislikes:
* We need more rituals!
* I'd really like to see racial choices make more difference to characters, whether in the forms of feats or powers only available to members of a certain race.
* 4e tends to be a little too tactical at times- I love me some minis combat, but 4e pulls me out of the game sometimes with talk of "squares", and precise minis movement.
* 4e needs more options for non-combat abilities and powers. Maybe giving PCs a non-combat utility power at the same levels they gain other utility powers?
* Good rules for healing and long-term injury. I understand what they were trying to do with healing surges, but it ends up feeling too "gamey".
* Some different way to manage powers other than at-will/encounter/daily. Again, a great idea, but probably not implemented in the best way.
* Although not a big problem for me, I can see why people would miss druids, bards, barbarians, half-orcs, gnomes, etc. This will be solved soon (or already has been, if you use Ari's Advanced Player's Guide), so its not as big of a deal.
* We need a viable OGL. During 3e, many of the best and most innovative products came from 3PPs. As of right now, that is one thing I miss with 4e, and I hope gets remedied soon.