In general, I approve of the role system, both for PCs and for monsters. It both improves transparency for users of the system (players and DMs) and helps to keep the designers focused.
That said, I have quibbles with a few specific elements. I think (and the design team has evidently come to agree with me) that the Controller role was poorly thought through; there should not be a role built around just having multi-target attacks. It makes the Controller heavily dependent on meeting a specific type of opposition - lots of minions and weak monsters - and is often boring to play. The new definition of the Controller as the "anti-Leader," the guy who debuffs and disrupts enemies, works a lot better for me.
I also think the Soldier role was badly designed. Soldiers were apparently intended to be the monster counterpart to Defenders, but instead of having "stickiness" abilities like PC Defenders have, they just have high defenses. High defenses should not be a monster selling point; Soldier monsters, particularly Soldier monsters that are higher-level than the PCs, contribute substantially to combat grind.
Minions are way too weak at high and even middling levels. They need a major upgrade. One DM I play under started hitting us with minions that could dish out a whole boatload of damage - something like 2d8+7 damage, per minion, at 16th level - and it was a huge improvement. We quickly came to have a lot of respect for those little bastards. Sure, they only had one hit point and you could blow them away like nothing, but if a few of them got up close to you, you were going down. (And let's not get started on the ones with ranged attacks...)
Solos, on the other hand, while their power level is fine, need a big versatility boost. As Vayden recently observed to me, the problem with solos is they're built, like all 4E monsters, on the 5-round model; a monster only needs 5 rounds' worth of attacks because that's all it's going to survive. But that's not true of solos, which have the hit points and defenses to last 10+ rounds; during which time they must provide a whole battle's worth of excitement all by themselves. They need to be more fox, less hedgehog. I think it should be a given that every solo has at least one "limit break" power, some new and unexpected ability that only comes online when the solo is bloodied.
Those are the roles I have problems with. The rest seem fine, and the overall concept is brilliant. (One thing - the Leader monster tag should not be merged into Controller. The Leader shtick can work on any monster and has nothing to do with what the Controller does.)