Over the years, I've dealt with a number of cloud based solutions and I've found a few lessons learned that I thought I'd share.
Cloud Software Privacy Concerns:
Not everything can be stored in a cloud, there are regulations and guidlines that may prevent information from being uploaded to a third party that can limit the use of software
Some cloud products just don't provide the intergration for local authentication to directory services, beware you don't want to be managing hundreds of accounts on a per cloud service.
Mostly Cloud based:
I've had some cloud based vendor that had some applications that still needed to be installed and those parts may just not work well or will require additional IT support. Some have activex or java requirements. Some need sync programs installed, very few were completely web based without some interaction with the os.