Some hosted wiki services do not limit the amount of data you may store at their service, but how much data can be transfered from and to their servers. This means the more popular your Wiki (having more visitors, creating more traffic) is the more likely to hit the bandwidth quota.

Especially when there's much activity during development or before deadlines, bandwidth can become an issue.

Also, if your wiki uses many images that change frequently (i.e. screenshots, product samples, sketches, mark-ups, etc.) or attachments that get updated often, the wiki may start to use a lot of bandwidth.

**What level should you look for?**
Many services indicate "UNLIMITED" or "NO LIMIT.  Obviously this should be your preference.