[mkdir]

Judging by the heatmap, it appears the author isn't familiar with Vim's c verb.

It works just like d, except it puts you in insert mode after you've completed your command.

For example, to replace the text within a set of quotation marks with You don't like Vim?, move your cursor on or within those quotation marks and use:

    ci"You don't like Vim?<esc>

To replace the text up to (but not including) the next exclamation point with I would never drink orange juice, use:

    ct!I would never drink orange juice<esc>

EDIT: Gah. He explicitly mentioned his use of cib. I'll leave this here in case it helps anyone.

[nbouscal]

Judging from his discussion of cib in the Data Acquisition section, it seems like he's familiar with it.

[falcolas]

Perhaps familiar, but not ingrained in his muscle memory to used instead of d<motion>i.

[nbouscal]

To be fair, I'll often do that as well, but just looking at the keys misses that there's a pause between the d<motion> and the i. I don't like sitting in insert mode, so if I don't know what I'm going to replace a thing with, I'd rather delete it, then insert something else after I've figured it out.