Here, hold my wallet for me.
For much the same reason I would never ask a complete stranger to hold my wallet, I would never allow my data to be completely controlled by someone else.
When I control my data I can guess at what issues I might have, and I can plan for them, but with my stuff in the cloud, I can only "hope" they are doing the same, I can never really know.
Things I would worry about are:
Environmental concerns (a la floods, earthquakes, and volcanoes).
Political issues leading to physically being isolated from the hardware storing my data.
Disgruntled employees destroying or copying my data.
Zero-day exploits on a holiday destroying/corrupting my data.
Backbone issues on the internet cutting me off from my data.
The company going out of business, and their assets being sold off with my data on it.
Criminal activity on the same server as my data, resulting in a government entity seizing my data in the process.
Their inability or non-willingness to improve their infrastructure creating incompatibilities with my hardware/software.
Future bandwidth caps imposed after the fact, prices going up at the end of the contracts. (for example your contract ends; they decide to bill per meg on outbound traffic, so trying to move my data to a new cloud service would cost hundreds of trillions of dollars. :)
Not knowing who really can see my data (sysadmins, various techs at the company)
Those are just the ones off the top of my head.
We in the IT fields need to shut this whole cloud idea down, or else there will be no jobs for us in the future, well except at the few cloud data centers (all located in cheap labor countries).