I've seen the background difference on old 1000-series NOOK 3Gs. My wife and I both have them, hers is on 1.5 and mine is on 1.4. In the Reading window, her background is darker than mine, but in other windows it's the same. Unless you're well-tuned to colors and shades, you'll probably need to have the two units side-by-side to see the difference (I did). It's not a big deal because IMO the bolder fonts more than compensate for the darker background, so the reading experience is improved in 1.5. But the slightly darker background is there.
Personal conjecture: remember 1.3 gave us fast page turns, but some units experienced "ghosting" where there were left-over bits from the previous page in a very light gray. 1.4 slowed the page turns back down to fix the ghosting. My guess is that B&N has gone back to the 1.3 fast page turns and is covering up any ghosting by setting the entire background of the reading window to a very light gray.
I haven't tried it myself, but somebody is complaining on the B&N forums that the page turns for Read In Store is "painfully slow" with 1.5. So apparently that's another place where the slower non-Reader page turns show up. I was stunned at how much slower my wife's 1.5 "My Library" page turns were than on my 1.4. Again, I had the two units side-by-side and clicked the page-forward (and back) buttons simultaneously. The 1.5 took (subjectively) about twice as long to turn the page, even though it had fewer entries in the library; btw, shelves were hidden during this test because no shelves were set up. In the Reading window, the reverse experienceó1.5 blew away 1.4, both forward and backward.