Hello! BC just changed room categories at the beginning of this year: It's now "standard," "garden" and "water." Before 2011, it was standard or water, so booking standard was risky. With the new categories, garden view means you should be overlooking a trees or some other kind of greenery. Standard means parking lot, rooftops, etc. So for an extra $6 - $10 per night, you can book garden view and avoid the parking lots and rooftops! It's not like other Disney resorts where "garden" is the least expensive bookable category.
As for the added cost of a water view, I think the water views are lovely. We've been upgraded to a water view in the past and really enjoyed it, but we wouldn't book that category - especially because the garden view category is an effective way of avoiding an unpleasant view.
Good luck with your decision!