The main difference is going to be, as everyone's said, no Scottish referendum - that means no huge SNP surge that wipes out everyone else in Scotland and it means English (and Welsh) voters not going Tory to keep out a feared Labour-SNP coalition. If the Lib Dems retain some seats from that, that's to Labour's benefit as (IIRC from a recent study) turns out many of their lost votes went Tory.
Without the SNP to the Tory's advantage, that leaves the economy and welfare. Election surveys found people still blamed Labour for the crash (even if they didn't like the Tories) and many think they're too much in favour of "scroungers". An earlier election means less economic recovery so maybe the former factor isn't as pressing but it'll still press hard. So I can see Labour still losing but it'd be a lot closer. And if it's a close-run thing, that may mean Miliband survives (but I doubt it) and more likely means his approach is still seen as the best chance, no big swing to Corbyn. So we're looking at Burnham or Cooper as leader.
The big question is UKIP. Two ways I see this playing out:
a) They won't have had any by-elections yet. This means the oh-so-earthshaking Rise Of UKIP that fizzled out, now doesn't happen at all - Cameron will still have nobbled them by promising a referendum, and if Carswell and Reckless still defect they're now running in a general election which makes their job harder. UKIP fizzle harder.
b) The SNP's rise hasn't fully kicked in yet, so UKIP sympathisers who voted Tory to Keep The Jocks Out are still going to go UKIP. Carswell and Reckless win as in OTL and so do one or two other candidates, so now UKIP isn't winning two by-elections and then losing an MP - it's going from 0 to 3 or 4, that's a thing to pay attention to. And since I don't see Farage winning if he couldn't win OTL (even with "JOCKS!" playing a part, he's the damn party leader), his position is screwed: he won't have a seat and here's some people in his own party with more legitimacy than him. So UKIP gets a new leader, potentially one harder to dismiss as a braying loon or potentially one that isn't as populist.
And then the Scottish referendum happens.
And that's a biggie because: the Tories just won again and just now, making "LOOK TORIES" an easy propaganda tool in the SNP's arsenal. Now I don't see Scotland voting for independence because of this. I definitely see the vote being a lot closer than it was in OTL.
That's a big problem for Cameron. Our Dave got to use the aftershocks of the referendum and the England-Scotland tensions to win seats in an upcoming election. This Dave has only just won an election and almost breaks up the union in his first few months, millions saying "I'd rather stop being British than be under him". He won't look very strong and it'll rattle him. That gives Labour an opening.
Minor note, to run as an MP, Boris will have to do it when he's only halfway into his mayoral term. That would look bad. Would he do it anyway and hope he can recover?