Fantasy gaming thieves guilds often tend toward a sameness--basically they're a sort of unusual trade union, as initially envision by Fritz Leiber in his Fafhrd and Gray Mouser stories. This was a fresh take on things when Leiber did it, but its time for a little more variety. After all, there are plenty of real world current and historical criminal organizations for inspiration, as well as some more recent fictional ones. Characteristics of these groups can certainly be used to add color to your usual fantasy world's thieves.
In looking at real world organizations, we find they often emerge among minority groups. Gangs formed in immigrant communities in the U.S., often for some degree of protection, and these developed into Irish and Jewish mobs, and the Chinese Tongs. The mafia didn't start in in an immigrant community, but the insular nature of these communities in the U.S. and elsewhere often allowed it to grow even more powerful. Even when the organizations' members are part of the larger culture, they many come from groups who are downtrodden for reasons other than just being poor. The yakuza, for instance, are thought to have formed from a combination of the trade organizations for gamblers and peddlers. Their ranks tended to be drawn from outcast enclaves where people who performed activities that were seen as "unclean" lived.
How often do fantasy thieves organizations have a distinct ethnic character--or at least, a history of a distinct ethnic character? Or how about if the make up of a thieves group reflected something interesting about the social stratification of the society in question?
Another common trait of real world criminal societies is that they function very much like other sorts of secret societies. They have esoteric rituals and customs beyond just an argot like thieves' cant. The yakuza traditionally had (or have) elaborate tattoos. The mafia, at least in fiction, has initiation rituals and distinctions between associate members and "made" men. Even the traditional trade union approach might suggest some sort of pseudo-masonic rites for a any self-respecting thieves guild.
Sometimes, criminal organizations have political agendas. Some, like the Chinese Triads, are thought to have formed in response to invasion. Certainly there are criminal enterprises in existence today with political axes to grind--though admittedly, these often cross the blurry line into terrorist organizations. Still, a thieves guild with that sort of ambiguous nature would be interesting, too.
Lastly, real world organized crime groups aren't monolithic, and there's no reason fantasy ones should be. Families or clans within a larger organization, make for intrigue and gang wars, and a lot of other fodder for adventuring. Are the families tighly controlled by a central authority, or is the peace more tenuous? Perhaps there are actually competing organizations in an area with different rituals, organizations, and backgrounds?
Those are my suggestions. If I had to pick a few resources to get the creative juices flowing, their certainly a lot a of "mob movies" worth seeing that give examples of how organized crime structures work. Most of Scorsese's mob films would do the trick, as would the Sopranos. To move away from the modern, the manga Lone Wolf & Club and the movies based on it give interesting vignettes on a lot of aspects of feudal Japanese culture, including the yakuza. In literature, Scott Lynch's recent book The Lies of Locke Lamora gives a lot of detail about the structure and ritual of the "Right People" of Camorr, an organization inspired no doubt by various Italian criminal societies, in the same way that Camorr is inspired by Venice.