We just upgraded to Jira 5.1.5 from 4.4.4 and while most users love the in-line editing a few are vehemently against it. They claim they are used to clicking and dragging on the page and now they end up making edits they didn't intend. While I personally think they will get used to it, it would be nice to make in-line editing a user preference vs only a global setting. A user wouldn't be able to turn it on from their preferences if it wasn't enabled globally but they should be able to turn it off if the global setting is turned on.
Note: This is required because some of your customers may be using tricks to control who can edit a field. Since Jira does not include field level security on who can edit anyone who is using custom screens via workflow actions would not be able to turn on in-line editing otherwise everyone with edit rights on the ticket would be able to change any visible field.