YAFFS is single-threaded, which would have been a bottleneck when trying to record those full HD video clips, and save them to the flash memory, whereas Ext4 doesn't have this limitation. Thus the new file system is more suited for usage with the multicore ARM-based chipsets that will be creeping into handsets and tablets next year. According to one Googler that has been pulled in to help with Ext4 for Android, as long as developers are mindful about how Ext4 does buffering and sync, there is no need to worry about data loss.
Ext4 will actually improve the handling of data loss, if developers make sure their application data is getting to persistent storage on time. Linux writes to so many files at once, that not all of them get written to permanent memory, and there is a risk for data loss. For example, when you drop your Android phone and the battery escapes, there is a chance those files that were not written in permanent memory, will evaporate.
There are newer Linux file systems out there, like Oracle's Btrfs, but Google went with the current generation Ext4 as more mature, not to mention that's what El Goog started using in its servers this year. It makes us all warm and fuzzy inside that Android got Google's server file system, and suddenly the little green robot can claim much more geek-cred with us.
source: Thunk.org via Arstechnica