Here are some useful system variables... punch them in on the command line. I know all of them work with AutoCAD 2005, and most if not all work with 2004.
mirrtext -> set to 0 (zero) to make text correct when mirroring linework with text
filedia -> set to 1 (one) if you hit open and you don't get a dialog box. (happened to me a couple times after upgrading to 2005)
peditaccept -> set to 1 (one) to get rid of the confirmation of turning lines and arcs into polylines, mostly with the pedit multiple command.
viewres -> set to fast zooms, and 20000 for circle zoom percent, to stop most arcs from getting the chamfered look when zoomed out. (i haven't had any performance slowdowns, but it is possible)
mbuttonpan -> THE KING! set to 1 (one) to pan when you press the mouse roller (middle button)
dblclkedit -> make sure its set to on if you want to double click a text to edit.
plinegen -> set to 1 (one) to make linetype generation for polylines automatically on. (good for plines with lots of vertices so you have the dashed line you want etc.)
The best for last!
mtjigstring -> set the default text (up to 10 characters) for mtext! see attachment! (good for other peoples computers at the office!)