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I'd agree with keeping it relatively simple, and multiclassing can lead to less powerful characters in one pillar, but not all three.
If you're a fighter, and multiclass rogue, you immediately should get a bump in the non-combat pillar of the game. This is where 4e failed miserably. In making combat taking so high a proportion of the game time, any optimization potential in your multiclassing rules inherently has to favour that pillar. (most of the skills we NEVER used, they were just too boring, and actually took the fun out of their use due to it being too simple and limiting, but I digress). But when you're talking about having fun optimizing your character, you're either into that or not, and if you are, you can be sure that any rules system will have optimization eigenvalues. Hopefully in DDN they won't all boil down to : find a way to get Twin Strike as your at-will, or simply "Be a revenant and win, all other choices are fluff after that"
I'd like a multiclassing system that's not front-loaded too, but I doubt that's gonna happen. Having to splurge your feats to another a level of something else...hmm, could work but the feat itself shouldn't be just a tax on multiclassing, it should confer something decent and worthwhile. We don't want a repeat of the 3.0 thing where every rogue takes one level of ranger, or the pathfinder other extreme where two weapon fighting is so feat intensive, you need your entire archetype / build to be set around it to be worth taking (and even then...all you end up with is a mess of dice rolls and varying combat bonuses that in practice are very annoying for other players to sit through...and probably you, too).
For me, if a system can account for multiclassing in chunks, like taking a PP or an ED, (how about every five levels instead of every 1). I know that at work, specialising will end you either in utter job security because you have an extremely sought-after set of (rare) skills and cannot be easily replaced, or with no job security at all because most people end up having to wear many hats. A fighter who can barely move is not much use out of combat, or even in dynamic combats, so taking a few levels or feats that allow faster movement while heavily armored SHOULD be worth taking. I like those "optimization" pitfalls that you may look at a sheet and say this guy is more optimized for combat, but will likely die because he has no self-healing or survival skills in the wilderness, or a high level wizard who can get smoked in one lucky sword chop or arrow (to the knee).
Not really sure where I'm going with this...sorry!! haha. But lots of good points in here. Let's see how it all shakes down. I hope that Aragorn is possible at various scales. Instead of Ranger in heroic, then Paladin as PP, then Warlord in epic, I'd like the option to build the same thing in a smaller level window, like levels 1-3 taking ranger, then 5-7 paladin, then the rest as some kind of mix. 4e did come up with a lot of good ways to break up class abilities with the hybrid system, though they are not all created equal (paladin's lay on hands, or channel divinity -- compared to armor+shields?-really --terrible. why even print it, it was insulting)