Let me start with a custom field that we have available on the issue level. We have a Bill As custom field to help us when we generate invoices. We generate all of our invoices with the data in JIRA as this contains all of our times via worklog. So an issue may be listed as Development - Billable, Development - Non-Billable, Support - Billable, Support - Non-Billable, etc...
What we've been finding though is that for a given issue, the majority of it might be Development - Billable, but the client may call up and as a quick question or so in relation to that ticket and we may want to log 15 minutes as Project Management - Non-Billable. That's not exactly the best example but I think it gets the point across.
It would be awesome if we could have a custom field available for the individual worklogs so we could set that per log that was entered. That would help us from having to adjust time after we generate the report to make the report look right and from having us to create a lot of other issues that relate to the one.
What would be even better is if that custom field could default to the main selection from the issue custom field. In other words if we have selected Development - Billable for the issue. The worklog custom field would default to that selection but could be changed. I'd be happy with just the first part though.