Last post I ranted some about the rut of quasi-medieval tolkiensque fantasy. I brought up the area of characters being terribly similar from campaign to campaign despite sometimes elaborate details in a given campaign. Here are a few ideas on breaking the mold through character races.
The strongest method of differentiating a campaign by the selection of character races is to simply drop the option. No character races at all save human does have an impact on play and the campaign. PC's don't get a host of pesky special abilities that can alter settign assumptions. No regular ability for anyone in the party to see in the dark (i.e. Infravision) and light sources are going to matter all the time. If the demi-human races are still present in the world but not as a player choice you have an NPC factor the players know well and will be able to deal with that doesn't require extensive explanation.
Reducing the number of available races. By cutting down on the number of available races a campaign is keeping some built in assumptions and special abilities but is making those abilities more important. Ultimately however, how different is a campaign where PC's can be Humans, Elves and Halflings all that different from one where PC's can also be Dwarves? Dwarves if they are excluded in a campaign as a player option but still exist in the game are likely going to be dealt with as more extreme stereo-types familiar to the whole fantasy RPG pastiche.
Race as culture. In this option Race isn't ones direct genetics but has more to do with the culture they were raised in. Culture having an impact on character differentiation is a useful tool that is touched on in fantasy RPGs but often underplayed or smushed up by having the non-human races represent a human culture. If one is a Viking in a quasi-medieval fantasy universe they most certainly are going to have a different set of basic skills and baggage to carry along when they meet others then say a Mediterranean Fisherman will. Cultures can be familiar or exotic but should have a lace in the campaign that has some value in building adventures and differentiation.
Race as Caste. In this option the traditional role of character race is that of the caste a character is born into. In some cultures this can be strictly defined with absolutely zero mobility in other cultures there is room for improvement or a change in caste. In a Caste system some character classes will be restricted to members of some classes or even the exclusive expression of that caste within the campaign. This will make for a world that is a fair bit different from the classical presentation of the pseudo-medieval tolkienesque pastiche. Real world society of the dark ages and middle ages could however be very rigid in what was expected of a serf, freeman, gentleman or clergyman. Campaigns with caste will be more alien then the traditional comfort zone of fantasy RPGs but castes themselves provide easy reference points for players to relate to.
Replace Races. Dump the old list of races (at least as PC options) and come up with some new races. There's an awful lot of room for fantasy races in fantasy RPG without having to use the same four or five over and over again. New races don't have to be brand spanking new in all of fantasy literature or D&D like gaming but a new mix of old tropes can have some impact. In my own one-off games of Swords & Wizardy I dumped all the normal non-human races and instead created one non-human race and two human cultures/sub-races as valid player choices (I went with Common Men, Amazons, Pygmies and Cyclops) to give the games a decidedly not-medieval setting. One should to write up these "new" replacement races as if they weren't going to be part of a campaign with the old classical mix.
Alien races. At first look the use of alien races seems just like replacing races. It isn;t really. Alien races are decidedly not the same breed of cat as replacement races. Alien races have more baggage to go with them and more mechanical differentiation from the each other. An alien race isn't just a person in a funny suit. Traditional RPG races can be recast as aliens and this adds a little excitement to old tropes but it also requires they be treated different within the campaign. A campaign with alien elves has could have them start life as flittering fairies that, as they get older, they lose the wings and grow larger and as they further mature they become dryads and treants, it's doubtful you'd have any half-elves and it's incredibly unlikely one would bump into an elfin serving maid at the local pub. Recasting familiar races as aliens can be difficult as player assumptions will pull them back to established stereotypes. Truly bizarre aliens can be hard to come to grips with because of the lack of familiarity but one has to question how familiar were folks with D&D-esque elves dwarfs and halflings during the early days of D&D? A campaign with true aliens has to be developed with these exceptions in mind or the alien-ness is simply forgotten or a barrier to play.
Dump mono-cultures. One of the largest annoying things about demi-human races is they often all are members of a mono-culture. A dwarf in one place is essentially the same as a dwarf in another place. By having cultural differentiation among non-human choices there are more opportunities to break the mold. Just stay the heck away from inventing Lake Elves, Moss Elves and Fern Elves.
Changing what race means or the selection of races can be very defining for a campaign but a campaign has to acknowledge these differences for it to work for the campaign. If guys A,B and C are really just treated the same as E,F and G there isn't much going on that's worth bothering with.