The mail and preview icons are totally out of place. They should have went with an iOS-like mail icon and made the contacts icon gray to match the app in iOS. Also, the current iOS iBooks icon should look like the one in Yosemite. Currently it is the same icon as the bookmarks icon in Safari, which makes no sense.
Yeah, the Mail icon has looked stale for awhile. I've never understood why they didn't at least update the stamp to coincide with the OS release imagery. Theres a certain amount of brand identity that goes along with icons, so I get whey they'd opt for tweaks over wholesale changes. But at this point, a tiny pic of Yosemite would make more sense than the random eagle that's been flying around there for years. Personally, I tend to swap my icons for custom versions anyway, so it doesn't matter much to me. But it would be nice if Apple would update the look every now and again, too.