I've been to the Polynesian 3 times (Rarotonga, Aoteroa, and Tokelau), and will visit once more this February. Each time I've booked a garden room. At Rarotonga and Tokelau our room looked out to the quiet pool and landscaping around the pool. At Aoteroa we had a 3rd floor room and could watch the monorail pass at some distance. The monorail was quiet and not noticeable with curtains drawn (my two sons loved seeing the monorail glide by).
A garden view room can have a view of landscaping PLUS just landscaping, OR a pool, the marina, a very blocked view of the lagoon, the monorail, another longhouse, and in one rare case a partial view of a driveway and parking lot.
I think the Polynesian's garden view rooms have the nicest "lowest price tier" views of all the Deluxe Resorts, with the greatest chance your view will be very very nice and a relatively low chance it will be less than the median. The least "garden like" garden view is arguably the one side of Rapa Nui where a car driveway and parking lot is visible beyond the landscaping. Note that Rapa Nui's has the bulk of the resorts handicap accessible rooms, and the driveway, portico and proximity parking is probably an intentional benefit for those guests.
Choosing a lagoon view will guarantee a view of the lagoon, but perhaps at some distance or through a small amount of foliage (think mostly lagoon to garden + lagoon).
Note you'll have to spring for a magic kingdom view, if you want to see the castle.