ON VANCIAN MAGIC:
I hate it. At least, I hate it as the core spell casting mechanic.
Each fantasy setting usually has its own story about the nature of magic and how one comes to wield it. I feel the Vancian system imposes too much narrative on what should be a very generic archetype (the caster of magical spells) for fantasy settings. Players and DMs should be able to make up their own narrative about what magic is in their setting and have an underlying core mechanic that supports it.
That said, I respect that others like Vancian magic and that taking it away from them is me accusing them of having "wrongbadfun." I also know its a D&D tradition that will never go away.
So here is a compromise that came to me. What if this is one of those "dials" in the core rules that each DM or group can determine for themselves (like they have done with healing in the current play test)?
SIMPLE SPELLCASTING: Use the 3E sorcerer mechanic - cast any spell you know as long as you have enough resources (i.e., spell slots) left to cast a spell of that magnitude.
MODERATE: Use the current DDN mechanic for clerics - prepare a limited number of spells, but then use your slots to cast any you have prepared in whatever combination as long as you have slots left.
COMPLEX: Classic Vancian Spellcasting.
I like the idea of spell slots as the core resource because it avoids any clunky or confusing shifts in paradigm you'd get from switching back and forth between spell points or mana.
If you wanted to really let each player have the option of setting the bar for his or her own character (and thus allow a simple and a complex caster to exist in the same campaign) you'd have to balance it out with some sort of trade off.
I don't like the idea that there would be two casters of the same class and level knowing a different number of spells per level or having different progressions based on the casting option they use (like was done with the wizard v. sorcerer in 3E).
Would the trade off be that as you got more restrictive, you would get additional slots, or perhaps your spells became more empowered with maybe different ranges, durations, areas of effect, or damage bonuses?
I would think this is best for wizards, clerics, and perhaps druids so you can play your old school classes on the core spellcasting mechanic. Newer additions (warlocks, sorcerers) could still have unique mechanics.