See ChangeLog for more details: https://github.com/joyent/node/wiki/Api-changes-between-v0.8-and-v0.10
Other than the Streams2 interface introduction I don't see much in the Change Log to be worried over. Most of the API changes are in features that (I think) won't see wide usage. But here's a couple high points:
In some cases where you use process.nextTick to dispatch work to the future, you should use process.setImmediate instead. Do this when your nextTick's are recursive.
Parsed URL objects will now have all fields, even for fields that are empty. Those fields will be set to null.
EventEmitter subclasses must now be done the correct way, by using util.inherits.
And of course there is the aforementioned Streams2 interface.