The storyline of this series is quite distinct. Unfortunately, the TV adaptation was discontinued, leaving fans attached to the characters. Perhaps more seasons would have been appreciated.
I watch a lot of series, usually multiple episodes each day. I haven't read any of the Mortal Instruments books, but I watched the movie and it was alright. This and the show getting the best drama series GLAAD award were the things that got me to give it a try.
The first few episodes weren't a total blast, but then it quickly became one of my favorite shows of all time. I look forward to each new episode and then never get disappointed. There are some emotional moments that make me tear up, the characters are all quite unique and pretty much spotless in their looks, plus a dark world of vampires, werewolves, elves and magi can get my attention.
I'm not a fan of the basic "guy gets the girl" kind of stories but despite the expected plot line where the girl bats her eyes and the guy acts all cool, it is yet to put me off in this case. Maybe it's that there are multiple more plot lines in the show, or that the characters are likable. The show doesn't shy away from sexualizing them, but yet again, rather than feeling forced and over-the-top, it serves more as a reflection of our world.
People need to stop comparing the show to the books, of course they're going to change things in the plot so it would still surprise the audience. I personally like the show, and I don't give a damm about the changes, if I wanted something like the books, I would just read the books.