My primary frustrations are with things that I didn't use in their earlier incarnation. The fixed position headers in Google+ take up screen space, giving almost a sense of claustrophobia for negligible benefit. Google Groups is painfully slow, and performing an advanced search (e.g., if you wish to search a certain date range) is currently not possible. Blogspot has a loading page.

For things that I have used, there are real, identifiable problems: the instant search results break the back button. Link tracking on search results means you cannot copy an URL.

I feel that there is a definite trend towards poor UI, irrespective of the user's prior history with the products.