Simple question?
Why is READ_COMMITTED_SNAPSHOT not on by default?
I'm guessing either backwards compatibility, performance, or both?
[Edit] Note that I'm interested in the effect relating to the READ_COMMITTED isolation level, and not the snapshot isolation level.
Why would this be a breaking-change, as it holds less locks, and still doesn't read non-committed rows?
Both. Mostly compatibility.
Turning snapshot on by default would break the vast majority of applications that expect the old, blocking, behavior. Snapshot makes heavy use of tempdb for the version store and its impact on performance is quite measurable.