From the wiki:
STORAGEFEE (5x) - per-byte fee for adding to contract storage. The storage fee is the only fee that is not paid to a miner, and is refunded when storage used by a contract is reduced or removed.
This would seem to incentivize storage use to the possible detriment of the overall system. If I get my STORAGEFEE back at the end, why not use it with abandon? It even might make sense to draw up a contract for the sole purpose of storing Wei in STORAGEFEE, which is not the intended use.
I realize this is all subject to change, but the tx fee structure is definitely breaking new crypto-currency ground and will need to be well thought out for the project to succeed.