I agree with tomaszd that getting to the point where some developers and early adopters can use Mer day-to-day would be a huge step. I don't think hardware support is the only way to achieve that. I think the minimal set of required features will vary for each user, and some people (like myself) may not use that much of the hardware. I mainly use my tablet for PIM functionality and browser connectivity. I gave up on fiddling with my n810's GPS a long time ago because it took too long to lock. Except for the occasional youtube video I watch on my n810, I wouldn't miss sound very much either. Bluetooth support is for me just a nice-to-have feature. Of course, I wouldn't give up these things unless Mer had some advantages to compensate.

I just installed Mer 0.16testing3 and have to say that it's not too far away from something I could use everyday. For my usage pattern, I think fixing the following issues would allow me to feel pretty comfortable with Mer:

1. screen calibration problem - doesn't seem to do anything, and without calibration it's hard to choose some small buttons
2. on-screen keyboard constantly popping up on my n810 - if I could disable it that would be a start since I always use the hardware keyboard
3. most icons not working - I installed gpe-todo and could hardly use it because the toolbar had completely empty icons
4. app manager crashes - the app manager seems very fragile - this makes it harder to try out some new apps. This can be worked around by using apt-get inside xterm, except the on-screen keyboard problem (#2) makes xterm very frustrating.
5. power management - I haven't had Mer running long enough to know if this is an issue, but it is important that my tablet can remain powered on in a standby state with light usage for a couple of days at a time. The default-installed cpu-meter worries me a little about this.

Aside from these issues, Mer has some advantages compared to diablo for me:

1. Mer's GTK fixes some issues with GtkTreeView expanders on diablo. I'm writing a little python app that depends heavily on GtkTreeView and it runs better on Mer (more like my ubuntu laptop).
2. I did "sudo apt-get install emacs22-gtk" and it just worked. I have a fully graphical emacs. It would be even better if it supported the full-screen hardware button, but it is very usable.
3. Mer's xterm and ls support colorization. Nice!
4. It looks like there are tons of apps ready to run that are just an apt-get install away, such as gnumeric. It doesn't respond to the hardware full-screen key, but it has a menu option for fullscreen. It also doesn't have the bug that occurs under diablo where you have to click on the edit bar when editing every cell, which makes it much more usable in Mer for me.