Warning: Keep away from children, pets, eyes, and DON'T mix with other chemicals.
I'm of the opinion that the use of AJAX technology can be great, but has it's own practical application, much like the cleaning product. When something intrudes on my web browsing experience, I get annoyed. Much like when I get AJAX on my favorite jeans!
An example of this is Snap Shots. It's one of those services that grabs thumbnails of websites and when you roll over a picture, or link, it will give you a view of what to expect when you click it. Interesting idea, intrusive implementation IMO.
Something else that gets me going is a website that has a login "dialog" which fades the content behind it, makes it untouchable, and throws a login form in my face(On the same vein, I hate that in an desktop environment as well). I understand that there are probably some usability experts out there that would say this is a good way to login to a site, because it doesn't take your attention from the content underneath as much as a page loads and multiple clicks would. I understand that, but if I'm ready to login, I'm probably done with what I'm looking at and am ready to move on.
Another thing I've noticed lately is the amount of JavaScript you see on sites with AJAX, chewing up resources on my browser, pissing off my Firebug. Do you _really_ need a massive library behind all your fancy AJAX when slim, trimmed down Codebases like mootools and jQuery already do 90% of the stuff people are trying to accomplish these days! These are just a few of the things that annoy me about this web2.0 AJAX thing.
P.S. Gmail 2.0 rocks.
