Moriarty never actually appears in the Holmes stories, he's just talked about (ie Watson never meets him), though there is a Paget illustration where he looks a bit like Mr Burns in the Simpsons.
He's more an idea really, a criminal mastermind protected at the centre of his web, that Holmes is so desperate to take down that he would die to do so. This incarnation was just incredibly irritating. Maybe he was actually just a front, and the real Moriarty hasn't actually appeared. Can but hope.
Overall, I think the writing hasn't quite held up to the acting. Cumberbatch and Freeman were great, and the chemistry between them was really good (I could actually believe the former was Holmes, which doesn't usually happen). However, although there were some good lines in the script, the plots just weren't very good. The last one was probably the best, but it all fell apart at the end a bit. The first episode in particular irritated me because it was so
obvious, and then Holmes was going round being all stupid "ooh, who can go everywhere but is never noticed..." He just wasn't clever enough - it doesn't work
having stuff the reader can get but Holmes doesn't, he has to be one step ahead of you as well as Watson! Plus there were some really lame bits where they tried to crowbar stuff in. Like replacing the watch with a mobile phone - "oh, it's all scratched where they were drunk trying to plug it in to charge..." rubbish!
That said, overall I pretty much enjoyed it. I didn't realise till after episode 2 that there were only 3 episodes, though. Why so short? I was expecting to get into it later on, but there wasn't enough of it to really get into!
"I've been in love with the morbid, ebony-black grotesqueness of the 19th century since I was knee-high to a funeral mute."