Nokia handset == multimedia computer
"Quite why Nokia users are the happiest ... perhaps ... or the interface and interaction experience is so much better on a Nokia."
Yes. Nokia's masterplan for all its higher-spec handsets is that they shall be very small multimedia computers capable of telephony, rather than cellphones capable of multimedia etc.
In my experience running Java applications is nicer on a Series60 phone than anywhere else. Thereafter the list runs exactly as per the user-satisfaction list in the article. Running Java applications on WindowsMobile based devices is pretty uncomfortable, which is unfortunate as such handsets are otherwise very capable.
Changing device settings -- likewise.
So I think that Nokia topped the survey because for anything over and above making a phone call, it's easiest to do it on a Nokia handset.