I've been using emacs regularly for around one year now, and I am always running into reminders that I need to consistently learn more about what's available to you in emacs. It's frankly a bit hard to schedule getting stuff done in emacs with learning more about what's available through (or in) emacs, but the return value for learning a bit more about what you can do is very high - there are things that can make you magnitudes more productive just sitting around waiting to be found.
A small tidbit, from earlier today: kmacro-insert-counter inserts the number of times a macro has been called into the buffer.
I was debugging some methods, and had a bunch of similar calls that I wanted to log info about to files named in a sequential order (foo1.txt, foo2.txt). I just needed to do a diff of the output. I made a little macro that used kmacro-insert-counter. Wasn't the most robust debugging solution, but then again I wasn't in need of one at the time, just needed to see some data as quickly as possible.
There have been a few other times where I've wanted something like kmacro-insert-counter. It's not something that's useful (to me) on a very regular basis, but when it's handy, it's really handy. Emacs is chock-full of stuff like that, and if you're familiar with what's there, combining all the little convieniences into one big project-or-file-specific convienience is easy, and allows you to do non-trivial editing tasks much faster than I've been able to accomplish with other editors.