I see a lot of people comparing the two OSs namely application wise. That is not entirely up to the OS. A lot of good apps can be written for any OS.
The main thing here is to provide the ease of use. Maemo on N8x0 is still quite Linux guru oriented, and if Nokia wants it to replace Symbian, they need to make it much more user friendly, even if they upset the Maemo purists in the process. For a lot of things, one still needs to open terminal and do things manually, which is something not acceptable for an ordinary user.
The other important issue is of course OS stability. I had a few situations when I needed to reflash my N810, without any apparent reason. This is something that must not happen to ordinary users. The system needs to be bulletproof, as Symbian OS is today.
Thirdly, seamless application installation, since Symbian is quite good at this, and I guess apt based package manager will be able to provide this. I haven't been that informed on application manager in Fremantle, I'm geussing it's still apt based.
Those are in my opinion three key points where Maemo needs to be improved, as it is now on N8x0 devices.