I just recently started a new game of Icewind Dale (which uses the same engine as BG2 for you people not in the know) and was thinking the same thing re: rules.
They really are best described as "ass-backwards". Thaco and AC starting at 10 and going down? Saving throws instead of DCs? Mages who start with 1 spell and gain roughly 1 per level, meaning they are completely fucking useless until bare minimum level 5 and really not on par with your average fighter until 10? Fighters having literally nothing to do except auto-attack? Exceptional strength?
It's hard to believe I ran campaigns for ten years using that ruleset. (albeit, a heavily modified version, but still.)
RE: fireballs
I discovered that even after not playing any infinity engine game for over 4 years I still have the explosion radius perfectly memorized to the point where I can place the fireball so the very outer edge strikes the line of enemies engaging my front line fighters but doesn't hit my front line
RE: Kensai
fuck yes Kensai are awesome (and also pretty much the best main character to have since in late-game BG expak AC means exactly dick)