AJAX vs Dialups
But please don't reach for AJAX too fast, because there is such a thing as being too interactive.
AJAX's interactivity comes through communication, and communication takes bandwidth. While it'd be nice if everyone coming to your web site had lots of bandwidth, it's not true (unless you want to make it true by driving away everyone else).
Let's take an example: using AJAX to implement incremental searches. The search box on your web pages uses AJAX to notice when I start typing and does a callback to your web server so it can show me matching results; once I've typed enough to pull up what I want, I can just go there.
So I start typing, entering 'p'. Lightning-fast, your highly interactive AJAX wakes up and sends the request back to your web server. Of course there are a lot of pages that match such a broad criteria, so the reply is not short (the RD light of my modem goes on solid). As I add a 'y' and a 't' the whole process repeats, possibly colliding with the data transfer for the initial 'p' in the process.
This hypothetical web site's great interactivity hasn't helped me, it's frustrated me. Search has turned into a laggy experience where I have to wait for the application to catch up to my typing. The slower a typist I am, the worse it may be; if I type fast I have at least a chance of outracing the AJAX over-interactivity.
So: don't be too interactive. If your AJAX needs results from your web server, you probably can't keep up with the user's interactions in real time. Don't try; wait a bit, let the user get a bit of a head start, give some feedback every so often, and reserve your big efforts for when the user has paused. (Pauses in user input are your big hint that the user is waiting for you now.)
Google Suggest shows another solution to this: don't return interactive results until they're small enough to be useful. (In a search interface I do ask that you put up some feedback to the effect of 'searching for "py": too many results to show in the sidebar', so that I can tell the difference between lots of results and no results.)
Whichever you choose, people on dialups (like me at home on my poky 28.8K PPP link) will thank you for considering them. And you may discover that there are more of us than you thought, along with the people using your web site from halfway around the world, the unfortunates stuck behind choked up corporate Internet links, and so on.
You can read about other AJAX design issues here and here. (And this entirely neglects the collection of practical issues one faces when implementing AJAX in the presence of network delays.) Note to self: AJAX is complicated in practice.
You can read more about AJAX in the Wikipedia article.