Well, in terms of who benefits the most from the new material I'm not so sure it is all just the players. Yes, there are a LOT more player options than there were before whilst the mechanics of the game haven't changed at all essentially (there are a few new areas that have been added but they mostly support new player options). From this perspective though it really just shows how basically solid the system is and how it managed to cover all the really important bases by using very generalizable mechanics from the start. I mean from that perspective what MORE do you really need on the "GM side?". Just basically more monsters, traps, etc which they HAVE supplied in good quantity.
However if you look at a lot of the supplements they are a DM goldmine. Open Grave, the Draconomicons, etc have tossed an enormous amount of plot material and easily adapted background stuff out there. DMG2 tossed out a LOT of good ideas for campaigns, alternate ways of doing rewards, etc. Even AV2 added a lot of interesting stuff that is likely to be highly useful to the DM even if it is basically player-oriented. Even just looking at the power books a lot of the player-oriented new stuff like beastmasters, familiars, domains, etc is really useful from a DM story-telling perspective as well since you can easily coopt most of it to use with NPCs.
As a DM I've certainly gained a lot from the newer material. I mostly don't use a lot of it as-is but its good material and has seeded some fun stuff into my game that I probably wouldn't have so easily done without having some of those books. Granted I COULD have done it all, but not as easily.
Overall its a good RPG. That's about all that can really be said. I don't think characters are more "flat" in 4e than they were in older Es either. In many ways they are more flexible. With a lot of really core character functionality in skills its MUCH easier for instance to set up a campaign centered around stealth and intrigue than before. The players can make pretty much standard style PCs and just take a feat each to be stealthy instead of being stuck only running "sneaky classes" (or in 3.x they can do it but only by doing some serious MCing and degrading their core competencies as a result). At worst such a 4e party would probably want to go with certain builds to be most effective, but those very builds would be thematic to that genre anyway (light armor dex fighters with rapiers and daggers, etc).