Slash'EM allows your character to be any of ten races: the five which exist in
vanilla Nethack (human, elf, dwarf, gnome and orc), and also
hobbit,
doppelganger,
drow,
lycanthrope and
vampire.
Hobbits are reasonably straightforward: they have infravision and the ability to "blink" occasionally (speed up for a while). Their strength can only get to 18 but their constitution can reach 20.
Doppelgangers have the ability to self-polymorph at the cost of some magical energy. Until they reach level 9, however, they are not always able to choose what form to take when they polymorph. (I am told that they have a better chance if they try to become monsters which they have eaten, but as they typically won't have eaten anything powerful, I have not tested this theory.)
Drow are good - whenever they hit a monster they can put it to sleep. This does not work on sleep-resistant monsters, but as air elementals are not sleep-resistant (for example), it is a VERY useful power. It more than makes up for their poor strength and constitution (which max out at 18 and 16 respectively).
Lycanthropes are rather annoying to play, as every so often they will randomly change into a wolf. Since a wolf can't carry nearly as much as a human, this isn't very useful, and when it happens I tend to change back as fast as possible. An amulet of unchanging or ring of polymorph control will prevent the change - if you can find one.
Vampires have good and bad points but on balance seem to be a difficult race to get very far with. (Also, until recently there were several bugs associated with them, such as being able to drain the blood from Death's corpse 4 times and without dying.) They have intrinsic regeneration, are immune to level drain and death rays, and do extra damage in combat by biting monsters and feeding off their blood. However, in their normal non-polymorphed form they cannot eat anything but blood drained from living monsters or very fresh corpses. So they often starve to death, and have great difficulty getting intrinsics. (When a vampire drains a corpse, he/she gains 1/5 of its nutrition and has 1/5 of any other race's chance of gaining any intrinsic the corpse has to offer. E.g. a vampire who drains the blood from a floating eye corpse has a probability of only 1/5 of gaining telepathy.)