I have not played it but have watched gameplay since release pretty much nonstop.
-People's biggest complaint is the circlepad. People seem to often do a side+B rather than up+B but this is mostly due to them not using 3DS very often. They said the controls feel awkward at first but you get used to it quickly. Each one can't wait for the GC controller though as they are used to that setup.
-Outlines are adjustable so don't worry about that. I haven't seen any gameplay where the outlines got in the way or were even very noticeable. I think it defaults as thin but if you have issues telling where characters are you can make them thicker.
-Online is generally very smooth. At best it works great, at worst people still say it's better than Brawl. However the key thing to remember is they are playing both japanese people and through japanese servers so lag is expected. You will have to wait for the US release for a true answer to this one. But I say if it works fine through Japan most of the time it'll be great when it gets here.
-Haven't seen one.
