In short: resolution, brightness and (not lcd-screen related) the speed of which you can zoom in and look at your images. I know dslr-comparisons are shunned upon here, but I don't see the problem with comparing it to my d700 here. My d700 has a very, very high resolution screen, and I can check critical focus 100% within 1-1.5 sec. Doing the same on the M9 takes 5-8 seconds and I don't even get to see if my focus is spot on or not, beacause the screen does not resolve enough. One might say that "well, if you shoot decicive moment photography, checking focus is a waste of brain/camera/time anyway", and that is true, but when i do concerts, portraits etc, being able to check if I got what i wanted, is important to me. And I'm a 90% manual shooter in the slr-world too.
My hope is that a future firmware release will make zoom-focus-check easier in the future. I've already started saving for a M9, so I hope that by the time I have the cash needed, it will be out of it's child diseases and into maturity 🙂