We successfully ran shared hosting for clients for years with CF Standard. The only thing we lacked was Sandbox Security. We managed all the sites, clients had no access to the servers, so Sandbox Security wasn't really an issue for that environment.
There are a few things involved with threading, such as PDF generation and the cfthread tag, that certainly gain from the Enterprise version. However, we never had any performance issues or complaints about "missing features" from clients over the 6 year period we used standard. I do recall those servers having an average of 150 sites per server.
There's nothing, aside from Sandbox Security (if your environemt requires it, and most do), that prevent someone from using Standard for shared hosting. It does boil down to what functionality you require, who your clients are, application architecture, etc. Likelihood is that you will need Enterprise for Sandbox security and also benefit from the performance enhancements in the Ent version.
Russ' points on Railo are spot on.