Google Sites is extremely underrated, especially if you're an individual and all you want is a wiki-style website. That is, something editable and somewhat dynamic, but more structured and without timestamps, without really worrying if it looks good. I use it extensively for private information sites and, of course, this site uses Google Sites. I do have a few annoyances with it, though I am very, very happy with its simplicity.
Scripting. I wish there was something better than having to use Google Spreadsheets (is that even right?) to create an Apps script that's useable in Google Sites. That said, scripting adds to the complexity of something I appreciate for its simplicity.
Moving content between sites is impossible.
Embedding content from one Google Site to another is impossible. Without scripting, evidently, but it should be OLE-style embedding. Remember OLE?
I wish linking to an external web address, rather than a page internal to the site, was the default when creating a new link, but only because I link to external sites more often than internally.
No integration between Google Sites and FeedBurner. It's possible to manually create an updates RSS feed, but it should be automatic, and the feed URL should replace the one in the <link> element on the update page. Not only that, but it should start with the FeedBurner domain I chose, in my case feeds.justagwailo.com.