I don't play much 4e, but I'm somewhat wary of its multiclassing system based on experience I had playing Guild Wars (yes, the MMO), which uses a similar system.
In GW, you have both a primary and a secondary profession. You can use powers from both professions, but you gain access to a special attribute to level up for your primary profession that you don't get for your secondary. The upshot is that a primary warrior is always better than a secondary warrior, because the former has leveled up his special warrior stat while the latter hasn't.
This all seems fine and dandy offhand. I started playing with a Warrior/Dervish, and all was well: my character was 60% warrior, 40% dervish. But, over the course of play, I discovered that I really liked playing as a dervish more, with a little bit of warrior thrown in. But, no matter how many dervish skills I used, I would never be as good as someone who had started as Dervish/Warrior.
Basically, I don't like the fact that one class has to be considered primary, and the fact that the class that is primary is fixed at character creation. What if my fighter starts to dabble in magic, and discovers he likes it more than fighting? In 3e-style multiclassing, I just keep taking more Wizard levels. But in 4e (and GW) style multiclassing, the best I can hope for is a 50/50 hybrid.