There's a trend on the net of using the term "LibLabCon". If you think those three parties are the same, you may as well add SNP to that list. They're as much part of the Social Liberal consensus of British politics as any of the main UK parties, and that's not a bad thing. The SNP favour a regulated free market with social security, healthcare and education provided by the state, just like the Lib Dems, Tories and Labour ¹.
It's the Green supporters that particularly confuse me by considering the SNP to be on their side in a way that LD/Labour/Conservative aren't. The SNP favour strong economic growth, low levels of business regulation, investment in road infrastructure, investment in the oil industry, and the EU. The Greens er… don't in all cases. About their only common ground is investment in renewable energy, which is not all that radical considering Scotland's potential there.
The Social Democrat faction are also somewhat naive in their view of the SNP. They focus on the lack of student fees² and prescription charges. However, they tend to ignore the SNP's proposals for lowering corporation tax, reducing business rates and reduce business regulation; which are really not that different to the Conservative's. Until 2008 Alex Salmond was pushing Scotland's future as a low tax, low regulation finance center in Edinburgh, looking to Iceland and Ireland and strongly supporting RBS's disastrous take over of ABN Amro.
All that said, the SNP aren't particularly bad, but like other parties they exist in a real world of compromise, dodgy lobbying, and past cock-ups. Other than their clearly distinctive policy on Scottish Independence, they're very similar to the general UK political consensus. Like the other parties they have their own distinctive set of emphases, but they're not fundamentally all that different to the others.
¹ Yes, I am saying that all major parties believe this, even the Conservatives. The Coalition haven't privatised the NHS, higher education is still available to all subject to exam grades rather than wealth, social security spending is currently at an all-time high and forecast cuts will take it down to 2009 levels, not eliminate it. That's all also annoying me, but not the real point of this post.
² The credit for that one is shared; in 2000 the Lib Dems in coalition with Labour initially replaced student fees with an endowment to be paid after graduation. The SNP abolished the endowment in 2008.