I would hate to see Scotland leave the Union, but I do think the Union requires significant reform to remain viable in the long-term.
The current situation in which most day-to-day decisions are devolved to the nations means that Westminster spends a lot of its time dealing with English issues - the English NHS, the English education system, the English prison system, etc.
This means that Westminster has become the de facto parliament of England, and the UK government is the de facto government of England. It is becoming increasingly difficult to imagine another Scottish prime minister, health secretary, education secretary, home secretary, etc. How and why would they be appointed, when most of their decisions only affect another nation?
However, Westminster and the UK Government do of course have very important powers in reserved matters, including most taxation and all of defence and (crucially) foreign affairs. But the situation I describe above means that these issues are decided, almost ex officio, by the ruling politicians in England.
We need somehow to carve out English decisions away from Union ones and to restore the connection felt by Scottish, Welsh and Northern Irish voters with the Union institutions.