Wulf Ratbane said:
If multiclassing carries NO penalty, then the game is essentially classless: a big heaping barrel of feats and powers than anyone can pick and choose freely from.
It's safe to say that multiclassing will carry SOME penalty. It will involve some form of informed, meaningful decision.
We can't really say that, though.
Say classes are build like they are in SWSE: class talents at odd levels, bonus feats at even levels.
Now, assume all classes have similar progressions, as it is rumored...thus, you improve your fighting capabilities through talents more than BAB; same goes for spellcasting: a "spell", or a set of "spells" is a talent you take (take a look at SWSE...force powers are gained through the "force training" feat...make it a class talent, et voilà).
Now, if you're a spellcaster, your talents make your spells more powerful ( for exemple, you have a talent that gives +1 to hit with spells, or another one that adds +1d6 damage to all your spells, etc...), similar to metamagic feats.
There is no specific "caster level", and your spell's effects are based on character level.
This way, multiclassing is easy: each class is a collection of abilities more than a career path.
Want to multiclass into fighter? Sure, take a level as a fighter, grab TWF and back into cleric. Your spellcasting capabilities are not hampered ( since they're based on character level ) and you got the talent you wanted.
Where's the catch, then? The "catch" is that you lost the opportunity to take a "spellcasting" talent, much like in 3e you'd have lost the opportunity to take, say, Divine Metamagic for that TWF feat.
The multiclassed character is not overpowered ( assuming all the classes are balanced ), and neither he's underpowered, since his spellcasting is not suffering a huge hit like it would have in 3.x, but it's still slightly worse than a "pure" spellcaster due to that missed +1 to hit or +1d6 damage.
Admittedly, it's all speculation, but it could well work this way.