I don't see any reason why not to allow unlimited power swapping, at the cost of a feat per swap. The most abusive combos I can think of involve a defender pumping themselves full of striker powers or controller powers, while benefiting from the defender's high hp and tank armor. But, this seems like a balanced option to me, considering the additional limitations on most striker powers, and the opportunity cost (the straight defender could be taking feats to supplement their damage output, and probably keep up with the defender/striker who is blowing all his feats on power swaps; and the straight striker is always going to do more damage anyway).
I think you're on to something here and I would immediately see Fighters becoming too powerful.
For example, the powerful Tanking Wizard Min/Max comes to mind. The problem is a Feat also allows you to qualify for Paragon classes. If a Fighter took several feats to eat up Wizard Spells, then went Blood Mage, they'd technically be better than a pure Wizard at his own game. They'd have more healing surges, higher hp, more proficiencies, and better armor. They would have more HP and HS to use the Blood Mage damage boost every round, while potentially having every Wizard Spell at their disposal.
Forcing players to split their powers evenly requires a greater distribution of power making it difficult (but not impossible) to cheese it up. Since most classes need two stats for optimal performance, it makes multiclassing difficult. But if you allowed cherry-picking then you could focus on classes that didn't have Stat requirements for key features, while taking powers and abilities from a class that was more beneficial.
Another example would be a Paladin that took all Warlock Star Powers. The Star Pact suffers from requiring a Con and Cha, neither of which benefits AC. The Paladin benefits from a Con and Cha and also has the best Armor prof in the game. As a Paladin you could now Mark at a distance and use Dire Radiance as an At Will. You'd dish out more damage with encounter and dailies as a general rule, while not having to fuss about the limitations of a Warlock. At higher levels you could take the feat that allows you to Curse two creatures at once giving you a greater spread of damage over an encounter.
I'm not trying to take a trip to Negative Town. Some of this can be done already but certainly not all of it. As it stands, a Fighter that Multiclasses with Cleric cannot currently take Flame Strike, Fire Storm, Astral Storm and still enter a Paragon class thus benefitting from the class features as well. Basically, deep multiclassing in 4e means you don't get Paragon class features. When you deep multiclass you're trying to craft a new build from scratch. Paragon classes offer a sort of "Package Deal" which is predetermined, less flexible for multiclassers and therefore more easily controlled. Whether or not it's balanced is easily stopped at the Paragon class itself rather than being systemic.
