Hey Falconfury - let me extend an early welcome aboard Deviate Delight to you! I will do what I can to summarize the most active RP guilds on both factions.
Others will no doubt follow suit, proving once again DD RPers are some of the most welcoming Classic players out there.
Short list follows:
For the Kaldorei, you have <Vengeance of Elune>, a top-notch raiding/RP guild with many active members.
For Dwarves, <Winterforge Clan> is your best bet. Another great group of folks, all-dwarf/gnome guild with good opportunities to raid, RP, and PvP.
On the horde side, should you decide to roll Tauren, look to the <Thunderhoof Tribe>. Great RP group, highly active in server events and fun shenanigans.
Though I don’t have a whole lot of experience with them (apart from many fun world PvP encounters with my Keepers), <Landlubbers> seem like a good bunch. They do events horde-side, and I see them around quite a bit.
Last but not least, the <Keepers of Stromgarde> are always happy to interview new soldiers, and we have a small but tight-knit roster of folks holding the line against the horde.