Guild Wars: Character slots in Factions
2 extra slots for a total of 6 if you have both GW: Prophecies (the original Guild Wars) and GW: Factions. If you're crazy theoretically you could keep the accounts separate and have 8 character slots. However, there's apparently 25+ new skills for each existing profession in Factions so not being able to use those on your existing characters, not to mention PvP characters, would be a terrible idea.
The way the skill situation will work is that there'll be a group of core skills that are available (more or less) in each standalone expansion. Each standalone then grants extra "optional" skills available to characters.