The one thing that most Thunderbird users miss is a complete and integrated calendar - well your wait is over if so.
Lightning has been timed to coincide with Sunbird and readily integrated into Thunderbird complete with organizer, task list, reminders, alerts and calendars. The look, feel and functionality are undoubtedly similar to Microsoft Outlook and likewise, you can add events and invite others to important meetings and events. However, for my money, the developers here have followed Google a little too closely. It feels very much like Google Calendar in use although admittedly, it's got more advanced scheduling and event detail options. Of course, Mozilla have taken much of Sunbird - their standalone calendar - and placed it in Lightning so if you're a Sunbird user, you'll find Lightening very easy to learn and pickup. The good thing is that Lightning is much lighter on your memory and of course, integrated perfectly into your e-mail so you don't have to operate two separate applications.
Sunbird, Google Calendar and Outlook users will find no major innovations in Lightening but it does bring a much needed add-on to Thunderbird.