#1: atime is important from an operations perspective for random usage file systems, such as tmp directories. Defaulting it off would make it counter to almost every file system that I can think of. [In fact, I can't think of -any- that default it off, but I'm sure there is one out there somewhere.] So a -1 on that idea.
#2 was done to mirror what we saw with posix, when fsck specifically hits a file (since fsck mainly works on files, not blocks, like 'real' fsck). I'm slightly concerned about changing this functionality, as I could see it being used during debugging (the only time lots of files are accessed at all is during a nightly fsck). But I recognize this is an extreme edge case.
#3 defeats the point of having atime at all.
#4 that just seems like a good idea in general. why hold it in memory if it isn't getting used? +1