If, say, you have a druid and a fighter, if both players are interested in combat to the same extent, both classes should allow them to contribute at about the same level of effectiveness over the course of several sessions; that should likewise be the case if both players are interested to the same extent in out-of-combat exploration or social interaction. They don't have to contribute in identical fashion, of course - if anything, you want the fighter to contribute in ways that are both generic and "fighter-y" and the druid to contribute in ways that are both generic and "druid-y", but if there's a real discrepancy in how the two characters end up contributing to the game over some span of multiple sessions, it ought IMO to come down to a difference in player skill or in player interest/engagement, rather than the classes' design.