Hmmm, can I get away with just a fighter and a thief with a bow or will I need a caster of some sort?
I edited my reply above, should help with this.
Also to consider is that everyone can do everything in terms of utility.
Everyone can steal effectively. If you spec into thief skills you get a few more options to steal with and a streamlined way of doing it, but you can plunder houses without a single point invested.
And everyone can use scrolls to cast spells, so if you want fire (for example) there's no need to roll a fire mage. Rangers have special arrows too, and you can use items like candles to set fires.
Resurrection is all done via scrolls, think Pheonix Down, and there are healing options without mages if you don't want to take a healer specifically.
Etc...