It does look like Apple gave with one hand and took away with the other in the case of Photos. Navigation in the main Photos (All Photos) section takes some getting used to, with the top left button showing more and more photos grouped in bigger groups, it takes a little experimenting to discover that you can go the other way by tapping on the photos in the group you want to see, or the title of the group if you want to see them by location.
The slide show appears to be limited to albums or shared photo streams. Definitely a loss. {I've pretty much eliminated iTunes synced albums on my iOS devices in favor of the new shared photo streams.}
What we gained was better/more editing options (including filters), and better shared photo stream management. The new grouping of the main photos tab is also pretty useful, once you get used to it.
I'm not sure what you mean by 'setting background correct size'. If you'd offer a more detailed explanation I'll take a look and see what I can figure out.
Many people who have been dissatisfied with the power and feature of the Photos app (from the beginning) choose to use third party photo apps. There are a huge number of these, and they cover pretty much anything you can think of to do with your photos. Photo Manager Pro is commonly recommended for those that just want a bit more control over how their photos are organized and displayed.
Me, I just use the Photos app, but I don't really need anything except a way to quickly move photos between apps, and to force people to look at pictures of my niece when I can corner them.
