I've been using this marvelous piece of software a while now, and find it
superior to all clients I've used before...in all respects but one.

I find myself missing a feature of Zmud, where you write a part of the
command, press up arrow, and it immediately searches for the last command you
typed with the same beginning. The new batclient just scrolls from the newest
command backwards. It's not that inconvenient, just a slight annoyance, but I
thought I'd voice my oppinion, and see if others thought the same.

Try right clicking on generic tab and then select Commandline -> Full line
tabcomplete. After that hitting tab should work just as you said.

Well, for me it doesn't seem to work. Or it does, but it seems a bit random.
If my last command was "use make reagent at ..." then typing u and pressing
tab will give me use mining at lode. If I write more than that one u, then it
only quesses up to the end of that word, and usually gets it wrong.
For example. use m -> use mining. use ma -> use make. use make re -> use make
read. use make reagent a -> use make reagent atlas...so no, not perfect.