I think it is effective. There were many times when I let a lot of email pile up in my inbox. Now, I very deliberately go through my email and respond to everything ASAP; everything that I can't do immediately gets filed away in a TODO list so I don't forget it. My inbox is thus ALWAYS empty. I don't know why but that makes me happy. It also makes other people happy because they are getting their responses faster. And, I get more done because having a list of "the next thing I need to do" makes me want to do things to get them off the list.
The other thing I found to be helpful is to schedule all my distractions. Now, I have a reminder that tells me "hey, check your email" and even "hey, check your feed reader." This seems silly, but it is really helpful; I know that I will get the "check your feed reader" reminder soon, so I just keep working until it comes. Sometimes, I get annoyed and refuse (!) to check my email because I'm busy; in fact, I had started with scheduling them hourly but I am slowly reducing it down to three times per day. As a result, I check my email and weblogs WAY less frequently than I did before (even with an hourly reminder), because before I would check my email basically in between each item on my TODO list or even in the middle of something.
I like a lot of things about Nozbe.com but it is too slow and I cannot do anything with it using my phone. Also, the fact that you will have to pay according to how many "projects" you have is crazy.
I'm trying very hard to make sure there's no UI clutter. I'll show my results by April 30th.
#3 and #4 mean a desktop app. I may consider that in the future but right now, web only. Sorry.
By #2/6 do you mean using AJAX?