One thing you shouldn't do is judge a show by its first few episodes because the show is just finding its footing. It needs to work out just what it wants to be and this is exceptionally hard when the show in question is an adaption of already existing work. The first few books of The Vampire Diaries series have very little in the way of story, meaning there was less for the writers of the TV series to pull from. They had a lot more freedom when it came to developing their versions of L.J Smith's characters making the show an adaption in name only. Saying that the first few episodes of The Vampire Diaries were slow and were more about character building before the real threat arrived. I'd even say the show didn't fully hit its stride until Season 2 so I went into Shadowhunters, a televised adaption of Cassandra Claire's The Mortal Instruments with an open mind. My first experience with The Mortal Instruments was from the film adaption starring Lily Collins and, despite the overwhelming negative reaction the film got, I throughly enjoyed it to the point where I was shocked that it had received even worst scores online than the Twilight films. Due to the underwhelming performance of City of Bones, the intended sequel was cancelled and instead the series was rebooted with a whole new cast and on the small screen.
With the advancements in technology, TV has become a real force to be reckoned with to the point where anything can look like a film in HD, and the nature of TV allows for more time to develop storylines and characters, making it a perfect platform for adapting a novel, but adaptions are tricky since it restricts the writers and directors in telling the story they want to make and any slight change can cause an uproar from the fanbase they're so desperately trying to impress. No matter what the source material is, whether it be A Song of Fire & Ice or Kelley Armstrong's Women of the Otherworld series, things are going to change. A character may be introduced as a substitute for two other characters or a character might even die when they had lived in the books. I also feel that certain books are easier to adapt than others.
Shadowhunters has the task of telling a story that has already been told both as a novel and as a film while still being interesting to watch. It's hard for me to describe my feelings towards the first two episodes because I know it has a lot going against it. It needs to prove itself to its fanbase while trying to bring in new audience members. It has to keep to the mythology detailed in the books but also keep things simple for those who haven't read the books.
The first two episodes are shaky. They try to keep the basic storyline of the book while also trying to add in its own elements but it feels like the show is trying to desperately get through the book's plot elements so it can move on to its own thing but this can create confusion to newcomers of the series. Characters react very briefly to what happens around them all in an effort to move the plot along. Movies have a limited runtime so they require the characters to move on while books can detail what a character is feeling without them uttering a word. A TV show has the time to show a character growing and responding to the changes around them but we're never shown it. Clary barely blinks in surprise when she arrives at the Institute and soon she's walking Simon in with a little smirk on her face at his surprised reaction to it all but even his reaction doesn't last long before he's part of the action! It doesn't help that his ignorance of the shadow-world being played more for laughs than proper world building. He only ever questions something when the plot require him not to be involved.
As far as production goes, the show looks nice. Aside from a few shoddy CGI shots there's not much to complain about. The show has done a far better job at showing magic in two episodes than TVD has managed in seven seasons. The sets, Pandemonium and the Institute in particular, are beautiful. Where the show fails on at the moment is writing, direction and acting. The dialogue, while serviceable, sometimes comes off feeling very forced or unnatural. It doesn't help that the acting of some of the main stars isn't very good. Dominic Sherwood, who plays Jace, hasn't shown nearly enough charisma to make Jace's character and sarcastic wit work. He berates Clary for not thanking him after saving her life just thirty seconds after doing it, making him come off more arrogant than probably intended, but this could also be due to the writing and the direction he is given. Again, the show is finding its footing so I expect this to get a lot better. The biggest offender is Emeraude Toubia playing Isabelle. Every flirtatious line that comes out of her mouth looks and sounds like its being acted by a pornstar. When she's given quieter moments her acting truly shines, but every time she flirts with Simon it just feels wrong.
Matthew Daddario as Alec is fine. The character isn't given much to do except scowl and make nasty remarks about or to Clary. Katherine McNamara as Clary is great but the shoddy dialogue proves even too much for her to sound believable. The best actor/character out of them all is Alberto Rosende as Simon. Every line intended to make me laugh worked and I look forward to seeing more of him. I'm not intending to be overly critical of the show or the actors as it is the early stages in the show. The direction of the episodes is OK, except for the action scenes that are either plagued by jumpy editing or weird motion blurs. I know TV episodes have less budget than a motion picture but shows like Teen Wolf and Game of Thrones have shown impressive fight scenes. Hell, even the Highlander TV Series had fantastic fight scenes (many I would argue are better than the fights shown in the actual movie series).
Episode two ends with Simon being kidnapped by vampires, an event that didn't happen in the movie until roughly half way through while removing Bane by having him go into hiding so the rest of Clary's memories will be hidden away from her. So far the show seems torn between its identity has an adaption and as its own story. It currently has thirteen episodes listed on Wikipedia but that might change depending on the ratings and the network's wishes. I don't expect the show to magically become a lot better in the next few episodes but I do expect it to be more focused and polished as time goes on.