MS Outlook has had something along these lines for years - they call it something else ("Quick Click" or something) but it allows you at least one-click colorization + task creation. It's not quite like he lays out but it's the basic idea of his "Actionsteps". I don't use it, and I would consider myself an Outlook power user. I have learned techniques that, quite frankly, just work better for me.
I use Outlook every day too, but don't really bother with the task tools that are built into it (even if it might (might!) integrate better with my email).
It's too general-purpose. It's like giving me a full toolbox, and the only limit is my mind! I'd rather have a tool that is more purpose-built to handle my needs.
These days I track todo-s in Trello and make sure my Inbox unread count stays at 0. That's it.
The only thing I'm missing is better attachment management, but that's just a nice-to-have. It's not a problem I personally run into very often. I could imagine though if your workflow involved lots of attachments it could be frustrating that they're treated as second-class citizens.
I use the heck out of the appts and tasks - appts are "things that I have to physical participate in" (phone call, lunch) and tasks are "reminders". I would not survive long without those. I use Trello but only to manage a small team of tasks.
You said, "The only thing I'm missing is better attachment management" - to me, the worst part about Outlook is the search functionality. It's just awful IMO. I can deal with the crappy attachment handling/saving/etc but when it takes a week for the "Instant Search" thing to index my 2gb Outlook file, I get pissed haha
