Currently Bitcoin-QT stores personal data (wallet, etc.) and blockchain in a user-specific directory (HOME-directory). Considering size and growth of the blockchain, it makes no sense to store a copy of the blockchain for each user on a multi-user system.
So I suggest to store the blockchain in a system-wide directory shared by all users.
Agreed on the problem.
However the solution to this is not to share directories, as as shared database is inefficient and tricky to coordinate, but to split up the client into a single "block chain daemon" and multiple "wallet daemons" that connect to the it on the same machine. (this has been the plan for a long time already)
How about just disabling the blockchain-support in Bitcoin-QT on multi-user systems/LANs and add a common blockchain-interface to bitcoind and Bitcoin-QT? That way Bitcoin-QT can use bitcoind as blockchain-daemon on a multi-user system/LAN.
It would be up to the user to choose whether to use a local blockchain (the default) or use a block chain daemon on the local PC or LAN.
Yes. On setup Bitcoin-QT can ask for the IP of a blockchain daemon. If none is provided, it just downloads the blockchain itself.
-blocksdir is now an option