After replacing Picasa Web with Google+ Photos in the navigation bar and redirecting individual albums to the Google+ interface, now Google redirects
picasaweb.google.com to the albums section of Google+ Photos. You can still go back to the old Picasa Web interface if you use this URL:
https://picasaweb.google.com/lh/myphotos?noredirect=1. Google displays this URL in a message that says: "Click here to go back to Picasa Web Albums".
When you use that URL, Google also sets a cookie value that prevents the redirect to Google+ Photos even when you enter
picasaweb.google.com in the address bar.
Google+ Photos has constantly improved last year, while Picasa Web Albums no longer added new features. It's obvious that Google+ Photos is an upgraded version of Picasa Web Albums, which will be discontinued in the future. There are still a lot of cool Picasa Web features that haven't been added to Google+ Photos (Creative Commons licensing, photo mapping, featured photos, search), so let's hope that Google won't retire Picasa Web Albums until Google+ Photos adds these features.
{ Thanks, Dave and Eric. }