You could use the favorite functionality instead, or assign a specific keyword to the photos you want to flag.
A hotkey for flagging is a whole lot different than selecting keywords and typing something in. I haven't looked at Photos because the fundamental premise, putting my library in the cloud, is ludicrous to me, especially given my super duper uverse max available upload speed of 1.5 Mbps. (Let's hear it for the awesome AT&T fiber solution!) At that rate, I don't think Photos will ever finish running, I can take more pictures than it can upload in a day.
I know there's supposed to be a local only version, I guess I'll get to it when I get there, I wonder how it's going to handle a library of tens of thousands of photos? Aperture has challenges with it, I get to relearn the whole managing your large library effectively all over again, I guess.
Honestly, it would have been better to have fixed Aperture than redesign the wheel, even if that effectively meant rebuilding the entire app. At least have a built in migration - Aperture frequently converted your library to the new version, and it was one way. This could have been something similar, as long as all the Aperture functionality was there.
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The competition being Windows Movie Maker
Premeire, Sony, and a couple of others. Movie Maker is iMovie's competition.