Congratulation on launching a killer feature once you addicted to it, very hard to move out from AWS :) (but honestly, this is a very awesome technology we've been dreaming for years)
If anything, their use of NFSv4 means there are plenty of competitive offerings if you decide that performance, security, or physical access constraints dictate migrating off their service.
If you don't want to manage your own Linux/BSD/etc. NFS infrastructure, Oracle, Netapp, and EMC will all happily sell you a storage appliance that supports it. I don't see much lock-in here.
I think parent meant it in a positive way, as opposed to negative. "So good you don't want to leave" as opposed to proprietary tech results in vendor lock-in.
