What people CONSTANTLY tend to forget that Thieves Guild works differently in every province. Here, they are part of Riften's mafia.
In Skyrim, they are for generations sponsored by the most powerful family in Riften and are practically allied with the Dark Brotherhood as that family likes to use them as well. They don't need to hide: anyone who tries to meddle with them gets their life screwed up. If Hyenormious Lex came to Riften, after three days he would get beaten to a pulp, lose his position, get his reputation slandered and just when he gets used to being a beggar, somebody slits his throat.
This is Skyrim, not Cyrodiil nor Vvardanfel. Different land, different cultures, different traditions. And I for one am glad that Bethesda tried to make something new rather than recycle them from game to game.
I haven't attempted to get into the Skyrim Theives Guild or the Dark Brotherhood yet. I'm hoping it involves a lot of zany hi-jinx, and not just killing people all the time.
Word of advice: learn the "Throw Voice" Shout. It makes sneaking a bit more fun and makes it easier to traverse locations with no minimal kills.