I just read an interesting article on ZDNet.com
Naturally it made me wonder why couldn't Opera run extensions? If extensions are really nothing more than JS/CSS/HTML what prevents them from being used?
I don't believe that would be useful.
I believe, extensions on mobile would be very useful. ;p
Keeping more limited resources (CPU time more than memory, as it's all about battery life) in mind, it should probably not be made too easy for wannabe-experts to install tons of them.
Extensions can preclude the sandbox and the whole debug system since we've seen a double amount of betas which relays to a remote. Bad idea to put a daemon inbetween, methinks.