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Arcane / Fighter gish types face a couple hurdles with the rules as-is. First off, you NEED a good AC now to be in front line, not just to fight off blows, but also enemy magic. So if you're gonna take hits, you need to be able to take them. How to accomplish that if you're supposed to cast spells one round and then back to melee the next?
Upon re-reading the Combat rules in "How to play".pdf, it seems that casting magic on the enemy you're currently engaged in does not risk disrupting it, so at least that part of the rules won't need special "hacks" (ahem), to make a functional Gish.
Whatever solution they come up with for MC, there should be "archetype" builds clearly suggested in the PHB so you can easily pick a workable combo. Such as multiclass + theme. So if your theme is more melee-oriented, you are more in-your-face type of gish, whereas perhaps an arcane archer, for example, could be modeled with another specialty combined with Fighter/Wizard multiclass.
I did think 4e managed to get the 1st level multiclass character working whereas other editions did not. Then again, the pathfinder solution was great, but required HUGE amounts of books to support all those classic archetypes. I think the DDN way is potentially better, but Wotc should take all their permutations and try to build all those archetypes out of them, test them out, and see if they work. If not, tweak specialties or add new ones. And ADD a "suggested builds" section in the PHB with GOOD choices. Ideally there would be no terrible ones, such as there are in every edition, but comparatively speaking, when I can make a better defenses "paladin" by reflavoring a fighter / BCL cleric in 4e, you know something's wrong.