I think that one factor that some may not be factoring in is that while the referendum saw a massive turnout (espicelly the young) it is not nearly as likely to be as big during the election and it tends to be the old who go out and vote. The SNP has done a superb job of appealing to the young but can they get them motivated to vote in a general election. Personally I doubt it. General elections just aren't as sexy as a vote to decide the very future or your country.
I think 40 seats would be a very good result for the SNP and it would certainly upset the apple cart in Westminster. Would they have enough to form a coalition with Labour though? Probably but it could be close.
I do find it funny though to hear the papers harp on about the SNP like they are the devil incarnate. They find it disgusting that a democratically elected party that takes part in britains democratic process should be allowed to help form a government. Perhaps the papers need reminding for how democracy works and that people can have a voice.
They're not going to go into a coalition with Labour, short of Labour offering proper devo-max which they will not do.
As to the youth vote, there's still an a palpable anger amongst Yes voters about how the Better Together conducted its campaign (that's somewhat lost in England, the local campaigning on both sides but primarily the Labour activist led BT campaign was pretty nippy). That's feeding into the SNP support for the polling, the question is will that anger be enough to get people to vote. I think the turnout will probably be higher in Scotland but not by that much. If the turnout in Scotland is really high then the SNP could get a lot of seats. If not they'll get a decent return.
The other thing which is harder for the opposition to admit is frankly even by the limited standards of what Holyrood can actually do (and in general governing in Scotland is pretty easy given you are basically spending pocket money), the SNP have been a pretty good government. The only blotches on their record are a Labour-esque centralisation of services (I have some sympathy here, 7 police forces were too much, but I'm not sure one massive one is the answer), a failure to find the magic bullet to get around , and the general performance of their (now ex) Justice Minster (the draconian anti-sectarian football law and a botched attempt to redo Scottish criminal law on the fly with the floating of removing corroboration as the underpinning of the system).
Ultimately the Scottish situation isn't much of a factor on the UK stage (other than what success the Tories get with their Labour/SNP scaremongering). Of the 55 you can safely say at least 45 of them (probably 52-53) will be an anti tory block in terms of the final Commons calculus.