I had a feeling I wouldn't be allowed to get away with such cursory remarks.
One of the first things I look out for in a writer is quality of writing - I've sat through some truly bewildering works like ER Eddisson's A Fish Dinner In Memison because I liked the texture of the writing. Banks is definitley a deft, skilfull writer, extremely good at immersing you in a totally alien setting, and at carrying out a novel-length experiment like Feersum Endjinn, narrated by a character who has totally strange spellings for nearly every word.
Coming to his actual stories - breadth of vision is the key here. Banks imagines events and conflicts on a huge scale and is able to portray them properly. Most of his SF novels deal with a galactic Culture, with the basic premise being that 'our currently dominant power systems cannot long survive in space; beyond a certain technological level a degree of anarchy is arguably inevitable and anyway preferable', to quote an essay by Banks. That's a debatable premise (all good premises are), but Banks' exploration of the issues inthe form of gripping stories is brilliant.
The Guardian descrbes him as a 'reactive social storyteller in the mode of Dickens, with a macabre strain comparable to early Ian McEwan.' That may refer more to his non-SF works (of which I have only read The Wasp Factory) but you can trace that sort of thing in his SF too.
I hope that was a little more helpful.