Allan at Dispatches from Paisley has written a very interesting post on 'where the SNP went wrong in May and how they can win next year'. I have to say I disagree with much of his diagnosis of the problem at the general election - while the dispute over the SNP's banning from the leaders' debates was an intensely frustrating quagmire, I don't think the party had much option but to do what they did. Granted, some people found it tiresome that they kicked up a fuss, but I think we can rest assured that those are the types of people who wouldn't have been minded to vote SNP (at least in a Westminster election) anyway. The danger of just meekly accepting the injustice would have been an even greater level of invisibility during the campaign, and indeed it could have been seen as a tacit acceptance of the charge that the party is "irrelevant" in UK-wide elections.
Allan is perhaps right to suggest that Labour's jibe about "Ripped-off Glasgow" had some effect, but I doubt it was huge, and in any case it would by definition have been restricted to one part of the country. As for the suggestion that the SNP chose the wrong narrative in "More Nats, Less Cuts" and should instead have been hammering home the issue of independence, I have my doubts. Again, there would have been a severe danger of seeming irrelevant, or at least detached from the campaign that people were tuning into - by focusing on the cuts the SNP were getting to the heart of what the election was really about for most voters. Perhaps one slight mistake was that the "protecting Scotland" angle seemed too parochial, or even selfish - they maybe should have emphasised to a greater extent that they planned to vote in Westminster to protect everyone against the worst of the cuts, not just Scots (as indeed is now demonstrably the case).
I'd suggest there's a danger of overthinking this - the SNP's disappointing showing (which wasn't half as bad as I feared at one point, incidentally) can largely be explained by the Labour juggernaut, which in turn can largely be explained by the false perception that only a vote for Labour was an authentic vote to keep the Tories out. How the SNP will ever overcome that inbuilt disadvantage in a Westminster election (short of fair TV coverage, I mean) I really don't know. In spite of the huge challenges that next May poses, at least we can be grateful that it's a "home fixture" for the SNP.