Even though it's not obvious, after you facilitate Google+ in your Google account, Picasa Web turns into a completely new app with a dissimilar set of rules. Some of these rules make Google's photo sharing service not viable for many existing users and it's important to know them in advance.
Picasa Web's face recognition feature helped you systematize your photos. The integration between Picasa Web and Google Contacts made it easy to connect your photos with some of your contacts. By the default, the name tags from your public albums were hidden, but you might also hide the name tags from unlisted albums. When you join Google+, all of this changes. Adding a name tag to a photo is no longer a personal action: your contact will get a announcement that you tagged him. He will get access to your photo and to the entire album that includes the photo.
"You'll receive an email letting you know you've been tagged in a photo. By default, name tags by people in your circles are regularly approved. You can view or remove tags at any time on the photos homepage in Google+ as well as the Photos tab on your Google profile," informs Google. Name tags change their visibility too: if you have entrée to an album, you can see all the name tags from that album. You're not the only one who can add tags to your photos: anyone in your wide network at Google+ (friends and friends of friends) can add tags.
While these changes could get better Google's face recognition software and agree to Google to add new social features, transforming Picasa Web's private tagging into Facebook's photo tagging is a radical shift. Sharing an entire album with someone just because you've added a tag is something that might baffle a lot of Picasa Web users who don't comprehend that Picasa Web is now Google+ Photos.
The good news is that the existing name tags remain unchanged and your contacts won't be able to see your albums just because you've tagged them at some point. But that's true only for the name tags added before joining Google+.
Now when you share an album with your contacts, they're permissible to re share it with other people, so you can no longer firmly control the visibility of an album without continually monitoring the access list. Google Docs has a similar rule, but you can change the sharing settings so that "only the owner can change the permissions." Google has an explanation: "to give self-assurance the natural flow of conversation, once you sign up for Google+, all albums can be re shared by people that have way in to the album - those people on the album's 'Shared with' list in Picasa Web Albums."
It's worth pointing out that the new Picasa Web adds some benefits (unlimited free storage for photos up to 2048 x 2048 pixels, easier photo sharing) and that you can wander your photos to a different Google Account, but it's sad to see that Picasa Web is now a Google+ app which no longer works well standalone and that users can no longer use superior features without sharing their photos.