For example, there are a set of commands that move the cursor in different ways (beginning of line, end of line, first line, last line, line #x, next word, previous word, etc. (that's ^/$/gg/G/g#/w/b) ), and a set of commands that performs actions (copy, cut mostly (y/d)). The real magic is how the two sets interact.
I don't have to memorize the sequence of keys to delete a word; I know the key to cut (d), and the key to move a word forward (w). I press d, then w, and the word is gone! It's a whole world of editing possibilities.
My secret shame: since I use GUI vims almost exclusively, I've never developed the muscle memory for hjkl movement; I use the arrow keys. When I started out, I knew i, esc, and that, and I figured out the rest as I needed it.
There's got to be a better reason to use vim than this. I mean...the standard for every other editor is ctrl-shift-left arrow, then the delete key and the word is gone! Whoopee.
It works in most editors, even Microsoft notepad and this textarea I'm typing in right now. I use unfashionable, clumsy old eclipse most of the time and could probably list 100 important things it does that vim never could. Things that actually matter. Maybe I'm wrong, but I doubt vim can launch and debug and continuously deploy to Apache tomcat. That sort of thing.
I've been using vim for 10 years and I learn new things all the time.
At that point I wouldn't start looking things up all the time. Just set out to learn one new command or shortcut every two days, applying it in your daily use. Usually that's enough to get them in your mid-term muscle memory, so that you continue to use when you move on to the next command. The Vim pocket guide is great to choose what's up next.
I was already fairly adept at Vim when I applied this method to teach myself less frequently used Vim commands.