Server downtime is not as much of an issue for unimportant files, or if you don't mind downloading them some other time if the server is down. A typical FWP will offer between 5 and 25 megs of space, with other offers of every description out there for the taking. Some will even offer unlimited bandwidth or disk space. There's no such thing as an unlimited account, and any account that stands out with an excessive usage of space or traffic will find itself squarely in the admins' crosshairs for investigation. Unless you've camoed those contraband files well, this normally means the sysops will throw a delete party in your account.
Turnover refers to the rate at which new users tend to replace old ones, as accounts get deleted.
A high rate of account turnover could mean users aren't particularly fond of the service and are leaving to seek better. It can also mean that the provider is zapping a lot of accounts for TOS violations or other reasons. You can get an idea of a particular server's turnover by asking around, or by checking out the general level of development of members' sites. The longer a site has been around, the more complex and developed it will be, much like a tree develops over time. A red flag should go off whenever the turnover is high.