I agree that they didn't go far enough. The game feels like it is missing something and at the same time like it holds back.
However, a lot of changes have been made to the game to the point that it doesn't feel like D&D at times. 3.x seemed sacred cow dependant, trying to hold onto as much as possible and this time they just about slaughtered the whole herd. Almost a "D&D, it's what's for dinner" mentality.
I'd like to have seen a doubling of at-will, encounter, and dailies. Utilities just give three more so the total for everything would be 4/8/8/10, doubling each should be a sufficient increase. When you run out of everything but the at-will the game starts getting boring cause you're doing the same thing without any real diversity. Also for the at-wills I would have liked to see them divided along this line, two from class, one from race, and the last one from the paragon, giving race another step-up in importance of selecting one.
I don't like that multiclass requires you to burn a feat to swap abilities and you don't get anything from it outside of the first feat which gives you a free skill, access to one of that class' abilities and you gain a benefit. You should be able choose which powers your character uses without burning the extra feats and gaining nothing beyond the swap ability. Feats are meant to enhance your character and the multiclass aspect doesn't do that. I'm waiting to see if it gets revamped in a later book, maybe PHBII.
Wizard’s spells should get progressively better as they level. If they increased the damage so that it rose by tier that would be enough I think. Look at the iconic spell Fireball; it no longer retains that usefulness as you level like it did before. All it would take to fix the spell would be to add: Increase damage to 6d6+ Int at level 15 and 8d6+ Int at level 25. I like that they gave wizards the ability to swap out their dailies, but hate that they are still limited to whatever their limit is. I'd like to see the wizard’s powers more in tune with previous editions so that they have access to more powers but still limited to the daily use. Wizard picking out two dailies is in a world of hurt if he picks two fire base spells and suddenly comes face to face with something immune or resistant to it. However if they could choose from more powers so that a wizard with two dailies could access four different spells that would be another story I am sure.
The skill system is a little weak also. Taking away the importance of skills tied to a specific class was a mistake I think though I understand the logic; however there were other ways to work around it. The best example here would be rogues no longer needed for traps and such. Removing the need for having a rogue in a party to disable traps seems less significant if you make them available to everyone to use, but they didn’t; instead they gave multiple classes the option of taking the skill related without any benefit. All it would take would be to increase the difficulty of a give challenge type by plus five and give those classes that either, gain a given skill as part of its features or choose it as a class skill, a plus five modifier. I know they already do this, but not exclusively to the point it reflects the need of having the given skills for a given class, increasing the difficulty of everything would balance that. The current progression and difficulty tends to allow parties to endure without a given skill. Another trend with skills in the challenge system is the introduction of challenges where the use of a given skill actually causes a detriment without any warning to the characters. As an example I refer to a challenge against a local lord who if intimidated automatically draws a failure or another where you might intimidate, but only after doing another skill first. These types of encounters penalize what could be justifiably role-played out in the game. I don’t care how powerful a person is, someone can find a way to intimidate them and there are multiple ways to intimidate. Seduction can be a form of intimidation for example as well as persuasion.
I also agree that as it stands now every class is the same. If you can play one class you can play them all. Some classes were an art form to be played by the right person when you watched or played at a table.
Overall I think the best steps to improve the game would have been to do more open beta testing, cut the levels in the PHB to 20, using the space to increase the number of powers available for those levels, and flesh out the differences between the races more.