As is Watson's use of the word "masochist": the film is set in 1891 and the word was first recorded in 1890.
Holmes' beat-up fedora is of the period, though just barely: the play the hat came from was first performed in 1882, and reached the English-speaking world in 1889 (the novel Trilby didn't come out until 1894). Even so, it took a while for the particular style Holmes is seen wearing to catch on as a men's fashion; early men's fedoras more closely resembled a Homburg◊ than anything else. Since everyone else is dressed much more traditionally, Holmes' wardrobe choices were probably made to showcase his eccentricity.
Dyeing for Your Art: Downey did this movie in between the two Iron Man movies. Look at how beefy Tony Stark is compared to how wire-thin Holmes is. That takes dedication.
Rob also was meticulous in making sure his accent was perfect for the character and historical period.
It was perfect for modern perceptions, but it wasn't historically accurate. Historical British sounded much closer to modern Americans than modern Britons. It mainly revolves around rhotic speech.
Combined with Continuity Nod, the Establishing Shot of Baker Street is almost exactly the same shot that opened the Granada Sherlock Holmes series with the seminal Jeremy Brett.
A rather far-fetched Shout Out is the newspaper a certain character reads at one point. The headline says "Panic in the streets." It's a London newspaper. Does Guy Ritchie like the Smiths?
"Dredger" bears some resemblance to Rondo Hatton, who played a similar hulking villain in the 1944 Holmes film "Pearl of Death".
The scene where Watson tries to interest Holmes in some seemingly bizarre cases that people have written in to him about, only for Holmes to curtly reveal he's deduced the actually-very-mundane solutions from simply reading the letters, harkens back to a similar scene in Billy Wilder's '"The Private Life Of Sherlock Holmes''.
A line from Henry V, Act III while leaving the cemetery: Watson: "Follow your spirit..." Holmes *joining*: "And upon this charge, cry God for Harry, England, and St. George!"
Of course, the most important part of this quote is the first line: "The game'safoot"...
Other films:
Hey, It's That Guy!: Both Grand Moff Tarkin (Peter Cushing) and Count Dooku (Christopher Lee) have portrayed the great detective with great acclaim in two separate occasions. Peter Cushing in particular has been considered amongst the best out of the more than 75 actors to have portrayed the character, which include the likes of Jeremy Brett, Basil Rathbone and Vasily Livanov, and was known to have an encyclopedic amount of Holmesian knowledge.
In the 2010 The Asylum film, Ianto Jones is Holmes' faithful friend, biographer, and personal physician.
I Am Not Spock: Basil Rathbone became perhaps the most famous actor for his portrayal of Holmes, usually with Nigel Bruce as Watson.
