Reliability: Generally speaking, data centers are backed up to the hilt. Some of the larger ones even back up to different geographical locations. Availability (hopefully 24/7) can also be important. Check for news of outages and the vendor’s own service blogs. If there are too many outages in service, buy accordingly. In truth, all the services we’re aware of are near-100-percent reliable.
Many services also offer a feature called versioning. This saves incremental changes you make to files as recoverable snapshots of the file. It's useful in case you need to get back information from an earlier version or if your latest file save becomes corrupted. Services vary widely in how many versions they keep and how long they're saved. Best-in-class services, such as SOS Online Backup and SpiderOak ONE keep an unlimited number of file versions forever.