I don't have d20 modern, but I recently read through the SRD. The idea of 6 base classes each based on an attribute is a pretty good idea. The game is pretty generic (although that could very well be because of the SRD), so the idea of more generic classes like that works. If its supposed to be for something more specific, like a CoC rpg (which only has 2 base classes, kinda pathetic) or d&d in the modern world (aka urban arcana) then I would agree that the classes are pretty lame and dull.
The wealth system is really interesting, but somewhat poorly executed. Lets say I want to have a very wealthy character, like that is my character concept. I even pick up Windfall and other feats like this. If I get my wealth up to 45, I could buy a tank and only take a 1 point hit to wealth... I could also buy a desert eagle and take a 1 point hit. Or I could pick up a +3 desert eagle of distance for the same 1 point hit.
One thing I found really annoying and stupid, Opportunist costs an action point why?!? Tough can get DR 3/- and its always active, yet this dnd-swiped ability costs an action point. The only way I could see this ability as a problem in Modern that isn't in D&D, getting to use Opportunist with your desert eagle on a guy your friend just popped, everyone being like 30+ feet from each other. However, this would mean AoOs work different in modern than in D&D, because you can't actually make an AoO unless you threaten the person (which ranged weapons don't). If that's the case, I can see the reason for the action point cost, but I saw no mention of AoOs working different.
Overall I feel the game isn't as balanced as d&d 3.0 and even less so compared to 3.5. I also didn't like how they made out NPCs as ultra non-heroic. D&D as been steering away from this, yet they backtracked with modern. I suppose this was to "explain" why PCs could do all these amasing things in a RL game (they are heroes), but I can't offhand remember anything in the base 6 classes that seemed unreal; all of the abilities are things you could imagine someone in the real world doing.