It does have a very high "bad language" count, but I disagree about it being "over the top". For the era and setting, it's entirely fitting. Furthermore, the writer has an excellent "ear" for when and where to use it, and for which characters. It's not at all gratuitous. For characters like, say, Swearengen or Jane or Trixie, it would be bizarrely out of character for them to *not* swear like they do.
There's even a certain "poetry" to the deft way in which the writer uses foul language for certain characters, it flows with the dialog and the character's mental state at the time, it doesn't feel as if it's just peppered at random into the lines like it does in some shows.
The same goes for the writer's use of "non-foul" language -- I can't recall any other TV show or movie that has ever had each character's lines be so tailored to their individual personalities and backgrounds -- they all use different vocabularities, manners of speech, styles, grammar, etc. fitting their characters. Unlike a lot of shows, a great deal of care has been taken to pay attention not just to *what* they say, but *how* they say it. It's another one of the many "attentions to detail" in "Deadwood" which make it an absolute joy to watch, and one of the best shows ever put on television.
I read recently that it may not be renewed because it's one of the most expensive shows on TV to make, so the producers are looking for other sources of funding besides HBO.