When it comes to deliberately turning off JavaScript, I'd wager to guess that the main three are security concerns, advertising fatigue and slow connectivity.
Security is actually very understandable. Almost every attack on a client machine happens using JavaScript (in most cases in conjunction with plugin vulnerabilities). Java of course is the biggest security hole at the moment but there is a lot of evil you can do with JavaScript via a vulnerable web site and unprotected or outdated browser and OS.
Slow connectivity is a very interesting one. Quite ironic -- if you think about it -- as most of what we use JavaScript for is to speed up the experience of our end users. One of the first use cases for JS was client side validation of forms to avoid unnecessary server roundtrips.
Now when you are on a very flaky connection (say a free wireless or bad 3G connectivity or at any web development conference) and you try to use for example Google Reader or Gmail you'll end up with half broken interfaces. If the flakiness gets caught during first load you actually get offered a "HTML only low version" that is very likely to work better.
Mycket, mycket bra och objektiv summering. Jag har själv börjat vackla i min egen övertygelse och känner att jag får ny styrka av att läsa det här. Nu orkar jag bemöta och avfärda alla som menar att de inte behöver ta hänsyn till avslagen eller strypt stöd för JavaScript i ytterligare några år.