In reply to lm610:
If you are talking about climbing on rock, then I think the only major differences are that the walk-ins are longer (so it's worth trimming all the luxuries and extras off your rack but taking a waterproof if the weather looks iffy, as it is further back to the car), the routes will be multipitch and you may need to climb with a sac on.
Multipitch isn't really any different to single pitch, but you might find that you need a few more quickdraws (particularly long ones and slings) and some more nuts (remember that they will be used in belays, so won't be available for the next pitch). It's a good idea to take a headtorch and a knowledge of how to get off the top of your route (a map and knowledge of how to use it can be handy on some crags, but not always necessary). Remember to take some snacks and drinks for belays on long routes, and a jacket.
The other thing that is different on mountain routes is that route finding is a useful skill - but don't get too hung up on the minutiae. Follow the obvious line that feels right for the grade, taking note of the obvious features. On popular routes, just follow the polish/clean rock.
In nice weather, I don't really think there are any differences between mountain routes and valley routes. It's when the weather is less favourable that things start to feel different.