There was an article on their site, which basically showed which faction level changes to which other.
The only factions which were lost (not converted) were two Burning Crusade ones, where the other faction had no equivalent.

Your current reputation with your own racial city will be the same level with that of your chosen new race on the other faction.
The other reps with your factions cities become the reps with the opposite factions other races.
Things like your honor with Honor Hold and Horde Thrallmar swap etc.

If you ever racial change back, you automatically go to your starting races.
There is no choice of what you end up with on your own faction.

As far as appearance goes, Draenei females are far and away the hottest.
Night Elves are decent, and humans are ok.
A lot of people like gnomes, because they're so small.

As far as racials go from a pvp point of view.
Gnome has escape artist, which is potentially the strongest.
The human has the pvp trinket as a racial, except that you might want a battlemaster trinket which shares the same cooldown anyway. Humans get a 10% boost to faction gains, which can be very huge as far as reputation goes.
Draenei get 1% hit and an instant cast HoT, which scales somewhat with your level.
Night Elves have Shadow Meld, which can be decent at times.

In general from a pvp perspective, horde racials blow alliance racials away.