fat, big portable
To me, that only thing that is a 'must' is selling the masses the concept of the Switch tablet itself: the fat tablet. Sure, some might not even want to use it as a portable so they will use it docked. No problem. But those that want a portable? Pretty tough sell, I think. Probably not taken further than the couch for most people, I think. The Switch can keep the TV free when going portable. A good thing for family relations, I suppose.
The fat tablet will be what millions of eyes will be seeing next year as it hits the stores. It will be the thing that either excites them or makes them shrug and turn away. Local multiplayer without fuss is always great. Buying one game that works on both portable and Home is good too. But will the compromises be worth it? Will they pull it off? I have some doubts about the perceptions of this machine, both as a 'portable' and as a weaker 'home' console than the Big Two. Nintendo can of course compete with its traditional weapon: low cost of manufacture, to prevent burying itself in debt. I bet we will see a Home-only version some years in the future too. This should be so cheap for Nintendo to manufacture compared to what Sony & Microsoft have now.
playing older games on Switch
Having older games play on this thing is a unique selling point for Nintendo. It's simply another way they can say they are different to tablets and phones & it would help sales for sure. Especially now they are really targeting the gaming fan rather than the casuals. The more unique advantages, the better. It makes sense to continue cashing in on nostalgia. Physical backward compatibility was successfully used to fight off competitors in the past, but I think it's far less needed today. What Nintendo needs to fight off today (with the Vita basically dead) is bad perceptions of the Wii/Wii U situation. Bad perceptions brought on by their own desire to save money on graphics grunt and their lack of third-party support for Wii U. I think this is their ball-and-chain now.
I think the typical user will be extra cautious when buying Nintendo hardware today (due to the above). If Nintendo don't deliver, I think this might be Game Over. A massively shrunken and more focused Nintendo company might be the result.