"Other" all over the place. Even the options I voted for come with qualifiers.
"All caster classes allowed but reduced number of spells" - full caster classes yes but drop a few half-casters, to the point where it's pretty much just Rangers (maybe) and Paladins as the half-casters. Also, instead of reduced number of spells (available) I'd go for reduced number of spells (castable in a day) and no at-wills of any kind.
"Spells require roll to cast properly with potentially dire consequences if miscast" - a spell cast under no duress shouldn't require a roll, and in many cases it shouldn't be possible to cast a spell at all while under duress e.g. in melee or while being jostled etc. The potentially dire consequences: an interrupted spell can (but doesn't always) cause a wild magic surge, with possible results limited only by how big a table you're willing to design/roll on for effects. And while the consequences of a WMS can be dire, they can also be highly beneficial...or, most of the time, simply amusing or inconsequential.
Oh, and area-effect spells should always require the caster to roll for aim, usually a fairly easy roll but if the spell doesn't go where you wanted you could easily hit your allies...or miss everything.
More broadly, magic becomes the high-risk high-reward limited-use option.
"Other" - there's no need to soft-limit the types of monsters encountered by labelling something as 'primary'; in fact I'd go the other way and enxourage DMs to mix up the monsters over the course of the campaign.
"Other" - implement a wounds-vitality or body-fatigue hit point system and make wound/body points much more difficult to recover or cure than vitality/fatigue points.
"Other" - find some way (and this would be new to D&D) of making it so a character at 1 hit point isn't as fully-functional as a character at half h.p. or at full h.p. The 4e 'bloodied' mechanic can be a starting point here, but with more gradations e.g. half h.p., 1/4 h.p., and 1/10 h.p. Intent: ultimately (and ideally) to make losing the first few hit points just as important as losing the last few.
Howzat for a start?
