Reasons to wait (a while):
1. Hardware upgrades required for our older Mini 2.1 computers (2 G RAM).
2. Rosetta is gone
3. Incompatibility with iOS VNC apps.
Reasons not to wait (past mid 2012):
1. No "free" iCloud without Lion
2. No iCloud at all without Lion (?)
3. Should be supported by most 3rd party apps by then including "versions" "auto save" and other new Lion features
I upgraded the RAM in one of our Minis and the hard drive got wiped.

I'm not sure how but there was no partition table on the thing after putting in the new RAM.

I restored from crashplan (Time Machine backup was corrupted).

From now on, crashplan is my main focus for backup and Time Machine is more of a convenience thing that I know I can't rely on.
As for not upgrading because I don't like the way it works, Lion is not that different from Snow Leopard and while it uses a bit more resources it is not a resource pig as far as I can tell so far.