Actually, I hadn't noticed this thread before either, and I am an avid "GTD" user.
For years I have looked for a tool or a combination of tools that would replace the DOS-based PIM (Personal Information Manager) Lotus Agenda, which provided me with a perfect GTD system since 1990. To make full use of the thing required you to have the capacity to be logical and able to think in terms of structured and complex cateqories, and it helped if you were a techo (familiar with computer technology), so it was not everybody's cup of tea.
Lotus Agenda's development ceased, and it was made progressively more obsolete by the newer Windows-based technology.
Which was why I actually became quite excited when I started to see the potential for Gmail + labels + filters to do some of the things that I had been accustomed to doing with Lotus Agenda. Some of those - but not all of them by any stretch - have been mentioned in the comments above.
A while back, I installed a Firefox add-on called "GTDInbox for Gmail". I found that by using this together with the add-on Better Gmail2, with the inbuilt categorisation tool "Folders for Gmail" (based on a Greasemonkey script) turned on, I was able to work towards roughly 60% of the power of Lotus Agenda - though it's a bit kludgy. You could experiment with these tools to assess how useful they could be for yourself. It requires setting up properly, in a structured manner, as did Lotus Agenda. I think the future for this Gmail-based approach to GTD is full of potential though.
I had one disappointment: I had been using an incredibly nifty and useful Greasemonkey script with Gmail called "Labellinks4gmail". It was a far more flexible categorisation tool than "Folders for Gmail"), but I had to use the latter because the "Labellinks4gmail" script was broken/defeated by the latest mods to Gmail.
Hope this makes sense and is of use to someone.