After much fruitful discussion and evolution, this proposal is now a series of "Bitcoin Improvement Proposals" -- see BIP 11, 12 and 13: https://en.bitcoin.it/wiki/Bitcoin_Improvement_Proposals
Sincere thanks to everybody who contributed improvements, ideas and code.
I like the idea of using highly optimized special-cased scripts for the most common cases.
And I think the optimized versions are much easier to read; they mostly look like the existing single-sig DUP HASH160. I don't see any advantage to generalizing.