As “Jesse says”:/comments/ruininguserexperience?page=1#1
He (and several others) mentions GMail as an example where a non-JS would hamstring you. As “John points out”:/comments/ruininguserexperience?page=1#8, GMail has a non-Ajax interface, but he brings up Google Docs as another example. I think that closed/subscription applications, like desktop applications, can justify a base level of requirements (think Basecamp too), as I say at the end of my discussion of Lala:
bq. For a closed application or service, this might be acceptable, but for a public website its a disaster.
The key here is _public website_ and I think “Chris’ ‘55 Chevy analogy”:/comments/ruininguserexperience?page=1#5 is a good one.
And finally, on the topics of NS4 and IE5/Mac, there are such simple little tricks to allow you to offer some modicum of support (typography, colors, etc.) to these browsers that it seems a shame not to do it. As long as it doesn’t cause conflicts with the modern browsers that are your target, I don’t see the harm. The experience won’t be “hi-fi,” but it’ll be something.
I often use @media=“screen,projection”@ on my linked stylesheets to hide layout-related CSS from NS4 and something as simple as wrapping the filename of an @import in single quotes instead of double will hide a stylesheet from IE5/Mac. The browser filtering eventually becomes just part of your process; in fact, my skeleton XHTML and CSS templates I use for every project are set up in this very way, so it is no extra work for me to offer this sort of staged support.