I was building a Survival MP world with 1.1, do I have to trash my old world and build a new one to get all the new features?
Nope, no jungle biomes or anything world-related for you (unless, possibly, if you go an explore an area that has yet to be rendered - by anyone), this may also mean no iron golems depending on how they spawn - also, if mountains / terrain can be generated above 128 blocks you will have none of that (except possibly unrendered areas as before).
As far as I saw flying around a world, the world generation doesn't ever surpass the old height limit to begin with.
Aww... that's a shame, I was looking forward to climbing a Mt. Everest or something.