I did a bit of searching, but couldn't find this question anywhere else.
Currently I am playing Icewind Dale 2 with a single character in the party - and it's a blast. This is only possible, because experience is split evenly between party members. So the fewer characters you have, the more experience each of them will get, making them stronger. This is the essential mechanic that makes it possible to play such a game with an arbitrary number of characters. For me, this is very likely the greatest replay value factor, also, because of it's implications.