By far I think the most over looked commands are the <C-x> commands while in insert mode. They do the auto complete. They all follow this pattern <C-x>letter.

Try out: l (line), n/p (next/previous instance of word), f (file)

Note that you can hammer these.

The next important thing I suggest is using buffers, splits, and marks.

Make sure you know your buffers and marks.

Also make sure you understand the difference between u/CTRL-R and g+/g-.

Macros (q) are also useful. Also, v, V, CTRL-V for selections. And gv to re-select previous selected area and gn / gN to operate on the next/previous match.

Star and hash - find previous and next match for word under cursor

n and N - find previous and next match for current search.