He was one of the showrunners, which meant he had final say on scripts that weren't written by him and also dictated the overall plot and character arcs of the series. He was responsible for this alongside Jeffery Liber and JJ Abrams.
Television episodes on US TV are written in a writers room, with many writers contributing to each episode, and then the writer who came up with the original idea, or wrote most of the episode is then credited as writer for that episode.
But as things stand, as a television writer, where the techniques and style are markedly different from film, he's not too bad.
How do I know the things I don't like about his writing are down to him? Lets look at the facts.
He wrote Prometheus. The idea for the film was by Jon Spaihts. If you look around the web you'll find his original script. Lindelof not only re-wrote it, but his inferior skills, ineffectual and poor dialogue writing, and penchant for assuming extra diagetic knowledge is a worthwhile endeavor in a film when a writer really should be providing more exposition, higher quality dialogue and not treating an entire film like it is simply another end of season cliffhanger are what made Prometheus so bad.
He was one of a triumvirate of terrible scribes who were responsible for Star Trek Into Darkness. Another film which is full of trite, cliched action movie dialogue, plot holes you could successful navigate the entire armada of Starfleet vessels through and boring set pieces (which are only actually good because of the director, cinematographer, editor and CGI team, not because of how they were written in the script).
He is also responsible for the final third of World War Z. Which is a COMPLETE tonal shift from the rest of the film, and is so full of awful dialogue that we actually get the rather brilliant Peter Capaldi say "He's walking right past them - he's just walking right past them!"; this is just one example of the completely inept third act of WWZ.
Granted, WWZ is just a poor film even before his section comes into, but he certainly didn't improve it by lending his hand to it.
So when it comes to film, he hasn't covered himself in glory.
He is a pretty decent TV writer, but that's because that format and medium allow for certain things that film doesn't allow for; you can employ cliffhangers, you can engineer characters and their backstory over the course of multiple episodes, allowing for deeper interaction between them and their environments, a TV writer is given more space and time to make characters believable. Some writers are more suited to it than others.
As a film writer, you can tell he doesn't know how to do that in a short space of time. That's why we get one dimensional characters in Prometheus, because he's not capable of rendering fully formed characters from scratch in the space of two hours, and seems to be quite poor it making us feel emotionally connected to them. He has had slightly more success on Star Trek Into Darkness and WWZ, but for the former he was dealing with two other writers and a bunch of character that we were already introduced to, and in the latter he was only employed to write the ending so really he had a lot less work to do.
My biggest issue is however that good writing doesn't just reflect our lives back to us like a mirror, it talks to us about something deeper, something unsaid, something knowing about the human condition which we feel deeply within ourselves. Lindelof's writing is like a mirror, it lacks depth. One can engineer emotional depth over the course of a season of television, but not every writer is blessed with the ability to say something meaningful about the human condition. When Lindelof is on his own, he's clearly one of those who lacks that gift.
