DD's assessment lines up well with my personal experience... which isn't too surprising since half of MY experience was pretty much the same as half of HIS experience!
Trees are more sparse & shorter in much of WCPP. The sections of WCPP we travelled this past year had their share of "vertical" terrain, though. Once you get past the fly-in lakes w/grandfathered resorts, it's all yours baby; there ain't much of anything else. The park exudes "wilderness". As mentioned campsites - where you can find them - are often ancient, with shrubs growing in the fire rings. Tent pads tend to be fewer & smaller. Canoe traffic off the beaten track is virtually non-existent. We spent 14 days there in late last June/early July. We saw NO other canoeists on the water (though there were some girls camped on our entry lake, Lund Lake, as we were leaving).
WCPP hasn't put in the same fishing restrictions as Quetico... yet. You can use your barbed hooks still. I think lake trout fishing is a good deal easier in WCPP than Quetico. However, if you like smallmouth fishing, best to stick w/Quetico.
While WCPP may not offer the variety of trees as Quetico, there are some striking differences in terms of the smaller flowering plants. There are definite influences from the western prairies. Also, thick carpets of luxuriant caribou seem more plentiful in WCPP.
I love both Quetico & WCPP. I'll be heading back to Quetico on an "old man's trip" this year... a kind of "milk run" trip. I'd characterize WCPP as more of a young man's park (ok... make that "young person's park"). You'll be roughing it more in WCPP. There seems to me more of sense of wilderness. It's a place that FEELS like something wild can happen at any moment and that you'd better keep your eyes open or you will miss that bear, caribou, or moose. Better keep an eye on the ridgeline across the water; you might just see a bear flipping over rocks looking for grubs. Also, you keep your eyes peeled for portages... otherwise you might not ever find them. They can be nearly invisible.
You can say much the same for portions of Quetico, too. Yet, in Quetico, there are times I feel a little more rushed about securing that decent campsite before some other paddling party beats me there. In WCPP, the feeling is more like you'd WELCOME seeing another paddler after several days of keeping your own company.
Both are great parks. Experience will vary according to what you are after and whom you are traveling with.
Jimbo