To be honest, what I'd find most useful is secondary DNS (at the right price

)
I'm currently using zoneedit for two domains (where I need a little more flexibility) and my old hosting provider handles the rest. As I move the zones over, secondary DNS becomes the issue, and either I'll either use my home linux gateway as the secondary, or have to start paying someone for the service.
I'll be using my linode for a handful of low-traffic sites (my own and a few friends) so will have a number of zones, but little traffic. Most of the zones will be 'plain vanilla', though my primary two have a number of extra A, CNAME and MX records (I have a couple of DSL lines, so several are duplicated/round-robin).
I'm not a big fan of arbitrary limits; if the potential for abuse is a concern, then monitor usage and handle the exceptions, rather than capping all users. I agree with irgeek - if the service is only for linode users, I doubt there'll be a problem.
Specific comments to irgeek's proposal: five domains and five A records wouldn't be sufficient for me. Five distinct IP addresses per zone would work though, so long as I could create multiple A records for each IP.
Paul