Vyvyan Basterd
Adventurer
Versatility vs. Power has never been that great of a dichotomy for balance.
If you choose a power from a class that synergizes with your chosen class, then it is well worth the cost of a single feat in 4E. Feats in 4E aren't worth as much as they were in 3E*. My contention is that the versatility has some ingrown power to it, so overall you aren't shorting your character in its power level (and even if you are losing power it is at most depriving yourself of 3 feats which doesn't put your character too far behind the power curve to the point of making a weak character). Obviously the majority disagree with me. My own players disagree with me. When I test my theory as a player I'll either prove them right or wrong. Maybe when that happens I'll start a thread here detailing my experiences with multiclassing.
*One example springs to mind. Not necessarily meant to prove that V=P, just a 3E to 4E example. In 3E you could do a whirlwind attak through feat choice. In 4E a non-fighter can gain the same option by MC into Fighter and chosing one of the Fighter "close burst 1" powers. Many people considered the cost of WA to be too high back then, but thought it a cool concept for their PC and still took it. MC now offers some of those same options, sometimes at a lower cost.