The new Marine Codex is powerful, yes, but overpowered? No. I can see a million and a one ways to beat their ass really badly. (Deepstriking Daemon Princes and Hive Tyrants come to mind, as well as plasma Plague Marine + Rhino spam and huge broods of ass-cheap Spinegaunts.) Also, the army just jumped from a beginner friendly to a lot more pro army. It has more rules than most other armies, making it really hard for beginners to remember them all or use some of the new complicated units efficiently. I can see new players starting Necrons and Chaos far more often.
Also, the new Codex makes Marines really expensive. The normal Tac Marines are a point more expensive than they used to be, which, while it doesn't seem much, really is. While they are able to wield some nasty combos, they are almost all so expensive the opponent can wield a lot more smaller and not quite so nasty combos at the same time.
Also, you have to have 10 dudes in a squad before you can take any special or heavy weapons. This is, however, pretty much not an issue, as you have the combat squads, but it can make a Marine players life pretty hard. For example, using the basic equipment, we have a 10-man squad of Marines with a Sarge (no equipment), a Flamer and a Missile Launcher. No double special weapons, like with Chaos. Now, the Marine player can, during deployment, either split these squads, which makes both of the parts really vulnerable, or keep them in one squad, which makes either the heavy weapon or the rest of the Marines pretty much useless. They either cannot move or cannot shoot the weapon.
Also, the much feared Drop Pods are not that special. I think they are pretty much just as useful as Rhinos (at least Chaos ones), as they are unable to move, thus useless for the squad they were carrying after they have deployed, can't tank shock or ram. The only real bonus they have is the ability to deep strike Dreadnoughts, but that is a pretty risky business, as bad rolls (despite the Inertial Guidance System) can take the Dreadnought somewhere the player really didn't want it to, for example, right in the front of a Wraithlord or into the firing lane of an enemy heavy weapons squad.
Also, as someone already said, the Marine Codex is the first strictly 5th Edition Codex. I am sure every one of the new Codices to follow will spawn a thread similar to this.