The design team on the show is absolutely fantastic, and they've included so many details I'm sure the fandom will be picking the series apart for years. While some of their choices are mundane (John's laptop ans his insistence upon the use of Internet Explorer), there are so many great design choices that deserve recognition.
That is a bison skull. Wearing headphones. I guess it hears no evil? This was installed in 221B before John arrived in Season 1, so we can only assume it belongs to Sherlock (unless Mrs. Hudson has a thing for odd collectibles). We have yet to find out where he got it, or why, but the mystery of the bison skull is almost as compelling as the detective's own cases. Pieces like this are so bizarre and macabre, they really capture the overall tone of the series.
Irene's phone, full of all her secrets. At first glance, her smartphone just looked like a regular blackberry, but of course something so ordinary wouldn't do for the infamous Woman. These phones are handmade with leather and titanum- and cist over $5,000 to start. That's not taking into consideration all of the mods she added to it. No wonder she didn't want Sherlock to have it.
The books! We all noticed this one because it ended uo being a pretty major clue in Season 2. But Sherlock's bookshelf also featured a staggering variey of books, from Becoming Evil: How Ordinary People Commit Genocide and Mass Murder by James E. Waller to The Adventures of Tom Sawyer by Mark Twain. Sherlock also keeps a collection of Guns and Ammo magazine in a corner of the flat.
I can only assume John brought the Sudokube home one day, hoping it would keep Sherlock from shooting at the walls for a while. Or perhaps it was another well-meaning but perhaps undesirable gift from a client, like? the cufflinks? Either way, Sherlock Holmes is many things, but mathematicin was never the highest on the list (we all know he really wanted to be a pirate).