Strangely enough the default keybindings you mention (C-j, C-., C-,) are all undefined in my version of Emacs (24.3.1).

--
Typed on a tiny keyboard. Sorry for being terse.

On 05.12.2013, at 03:05, Stefan Monnier <monnier@iro.umontreal.ca> wrote:

>>> 1. How to pick the first item on the list without typing a lot? I want
>>> to type a substring RET to pick the most recent buffer. With
>>> icomplete-mode I type t e s t TAB and get test*.
>> Why type TAB?
>
> Oh, I think I understand. You should hit C-j to select the
> first item. If you prefer TAB, then do
>
> (define-key icomplete-minibuffer-map [?\t]
> 'minibuffer-force-complete-and-exit)
>
>>> 2. How to quickly rotate through the list? I want to type t e s t C-s
>> C-. and C-, (the logic behind those is that , and . are just below <
>> and > in the US keyboard).
>
> Of course, you can also use C-s and C-r with something like:
>
> (define-key icomplete-minibuffer-map [?\C-s]
> 'icomplete-forward-completions)
> (define-key icomplete-minibuffer-map [?\C-r]
> 'icomplete-backward-completions)
>
>
> -- Stefan
>