I'll admit it; I'm slow sometimes. For a very long time now I haven't really gotten why people keep sprinkling AJAX over their web pages (partly because I assiduously use NoScript and so mostly don't see it). Oh, I understood that you needed it to create actual applications on the web and that it could be convenient for making vaguely friendly things, but I didn't really understand it in the context of relatively ordinary web apps like DWiki.

But my recent thinking about my comment form design mistake has finally fixed that. Here is my recent insight in a nutshell:

AJAX lets you do things without page changes and refreshes, so you can preserve the user's context on the page and make them less confused.

In a conventional non-AJAX web interface, any significant action forces a (full) page reload. This creates a visible page refresh except in extremely ideal circumstances and in general means that the user has to find their place again and reorient themselves. This is sort of tolerable if what the user is working on fits entirely inside their browser window; it's fairly horrible if it doesn't and they have to actively scroll around to find where they were before. This is the core problem I have with a revision to my comment form design; I'm pretty sure that people would get lost among everything else going on.

(The ideal circumstances are that you're using fragment identifiers in the URL, the browser accurately repositions things back at the fragment identifier, and the entire system loads the new page so fast that there is no visible flicker.)

In an AJAX web interface the user can perform actions without this lurching jump. For example, when they click on 'add comments', they don't get yanked to a new page; instead, a comment form unfolds right then and there in front of them. This is less confusing in two ways. First, it is happening right in front of you, clearly visible. Second, it is the only thing that is happening; you don't have to pick out the significant change from all of the other flickering and movement and so on that's going on as the page reloads.

This creates a more fluid, less disorienting interface, one that is easier and faster to work with because you spend more time doing what you're interested in and less time finding your place again every so often. In a sense, the result is much closer to a direct manipulation interface than a standard, non-AJAX web page can manage.

I don't think that there's any way to pull this off without AJAX; you really need some way to do a partial page content update without anything else flickering or moving. That's just not something that browsers offer (you don't even get it on plain user-initiated page refresh).

(I suspect that this is old hat for people in the field, but all of it only clicked for me when I started really thinking over the problem of people getting lost in my comment form under various circumstances, cf TemplateLimitations.)

PS: looking backwards, this makes me slightly more sympathetic to old HTML frames. Although they were almost never used this way, you can argue that they were a crude first attempt at the sort of limited page update you'd need to pull this off.