* not completely decentralized or open source. If wuala goes out of business, your data may not be recoverable.
* web interface is lacking (poor folder navigation/listing)
* doesn't work without X on linux
* even with X, not all features are available through the command line or API, though some are
* the interface that is provided is clunky
* the status messages leave me wondering where in the process an update is. If a piece of software can't reliably tell me where it is in a process, I can't trust that the process is happening the way I expect.
I suppose Linux definitely isn't their main market and while the Web Interface is lacking at the moment they are working on an overhaul.
Concerning it not being completely decentralized - I consider it a plus since that fact ensures greater reliability.