This addresses theymos' concern that senders shouldn't be burdened with extra fees from longer scriptPubKeys. Instead, for more complex transactions, the scriptSig is longer which means that the owner of the address bears the cost of potentially increased fees.[/li][/list]
Actually, I am quite disturbed with implications of shifting the burden of fees to the receiver (as transaction fees are at least partially dependent on conditions that only the sender is exposed to, such as for instance whether the sender's balance is made up of a bajillion of tiny 0.5 BTC inputs)
I'll PM theymos so that he can convince me without turning this topic into a rehash of something he might have already discussed and explained in detail
Addresses for arbitraritly complex transactions are fixed forever. No more new address types need be introduced.
In my very humble opinion, giving "new and exciting" types of BTC transactions a different address type (either as "catch-all" for all innovative transactions or have a separate address type for each "officially supported" new transaction type,) to avoid confusion and help people grok new and fairly complicated functionality more easily.
Methinks that different addresses for exotic transactions will actually increase usability, especially for "the kind of person" who only lives in GUI and won't touch the user manual
unless ABSOLUTELY necessary