I am reluctant to move to Outlook (even though I have it as part of Office 2011) mostly because, although it would solve this feature downgrade problem, it is rather bloated and doesn't intergrate well with sync services. I've just downloaded a free trial of the, on the face of it, overpriced Busycal (you can get it from the App store, too). Busycal doesn't mean that you can create to dos from within mail, but you can drag mails to its to do panel, which you can't do from Lion mail to lion ical (unless you drag it first to the calendar and then drag it again to the reminders).
I've also noticed a number of other featues that make it more useable than the new iCal. It also syncs with all the sync services stuff including, incidentally iCal, so I plan to try it for a little longer before deciding what to do. Who knows Apple may even be planning to add some to do functioanlity back to mail or release a proper scheduling facility as part of iCloud. Please do send them some feedback about your disapointment http://www.apple.com/feedback/mail.html becasue there's no reason to think they'll be tracking this thread unless the number of views and comments grows to something embarrassing.