He's one of the most recognizeable characters of all time. Sherlock Holmes has appeared in more films than any other character (unless you're counting Draculas, but we all know Draculas aren't people). Perhaps it is because of the character's noteriety that we've forgotten so much abiut him. For example...
He died (and was supposed to stay dead)
Sir Arthur Conan Doyle was far from Sherlock's biggest fan. Doyle was more interested in writing historical fiction, because he felt it was more important. So Doyle created Moriarty, a villain clever enough to take down the brilliant detective. The Reichenbach Falls was intended to be the end of it. His readers went ballistic. They cancelked their subscriptions to The Strand (where the stories has been published), wrote hate mail, wore black arm bands to express their state of mourning over the detective, and wrote petitions to bring him back. (via Diane Duane). Doyle had no choice but to hring him back.
Sir Arthur Conan Doyle inspired Jurassic Park
Remember those historical novels I mentioned that Doyle wrote? 'The Lost World' was not one of them. Doyle wrote an adventure story featuring a South American country where prehistoric animals still lived, and that story was the inspiration for Michael Crichton's book of the same name, which led to the Jurrasic World stories we know today. (Sir Arthur Conan Doyle is basically responsible for everything you love).
His first film appearance was in 1900
The short film titled ' Sherlock Holmes Baffled' does what it says on the tin. The film doesn't resemble any original Holmes stories, so it's safe to say the production company was trying to capitalize on the character's popularity. To give you some perspective, the last Sherlock Holmes story written by Doyle was published in 1927. Imagine being a writer during this period and seeing your character on film though. While not uncommon today (with video cameras widely accessible), that seems like it would have been mind-blowing at the time (even if Doyle hated the character).
He never said 'It's elementary my dear Watson'
At least, not until Basil Rathbone played the character onscreen. Yes, one of the most famous quotes attributed to the character was not even in the original stories. This is why we cite.
His name was originally going to be Sherrinford
I know Sherlock isn't exactly the catchiest name, but Sherrinford definitely sounds worse. Thank goodness he changed his mind.