One of my favourite bits of DMing advice comes, in all places, in the 2nd edition DMG. Zeb Cook (I am paraphrasing because I don't have it to hand) basically foreshadows some OSR thinking in one place only, which comes in his discussion of stat requirements for character classes. Don't let players access any class they want, he recommends; keep the stat requirements for rangers, paladins and so forth strictly. Encourage players to play the stats they are given, so to speak. Let the dice fall where they lay; so your PC didn't end up getting the 17 CHA required to be a paladin. Stop being titty-lipped about it - you can play a fighter who always wanted to be paladin but failed (or hasn't yet succeeded).
In other words, a fighter who wants to be paladin is already way more interesting than a paladin. That's a PC with a ready-made goal and pretty much ready-made personality too. He may not have special paladin powers but being "awesome" isn't what the game is really about. It is more about trying hard and applying yourself and getting involved.
You can tell 2nd edition was very much focused at a younger audience - this is the kind of advice that's important for kids, whereas adults should in theory at least have already learned those lessons. But the wider point holds for all ages: it's good for starting PCs to have goals. It makes the PC part of the world and gives the player an impetus to drive things along, which helps the DM tremendously.