Has anyone found any guidance on developing for Office 365/SharePoint Online based on the SharePoint 2013 product line? We have been tasked with upgrading a webpart that was developed for SP on premise, and migrating it to Office 365/SharePoint Online. The solution was implemented via the SP API, on premise SharePoint 2010. I know we're looking at implementing it as a sandboxed solution, and that was the case when Office 365 was SP2010 based, but, what's different now that people are being upgraded to SP2013 based SharePoint? Thanks!
Sandboxed solution are actually deprecated for 2013 (Source)
Have you looked in to the new App model? This is Microsofts new preferred way of developing for 365, and also partly for on premises.
Here is a comparision between the App model and "normal" SharePoint Solutions