To start, this article isn't by any means bashing the "Throne of Glass series," Sarah J. Maas, or anyone who enjoys either of those things! This is just my personal opinion on the books in accordance with my tastes.
When I read the first book, which the series is named after, I didn't know it was part of a series yet. I was interested enough to get through the whole book, but at the same time I didn't really feel the urge to buy the next book to find out what happened next. It captured my interest, but there were plot points the main character could have easily figured out that were saved until the very end to be some surprise reveal. They were things that the reader themselves could figure out, so it seemed off that the main character didn't.
The main character is also a bit odd as a heroine, but in this case, I applaud the author for not sticking to what's expected out of the main character. Celaena's personality is very distinct and did get on my nerves sometimes, but I liked that she was more dynamic. I liked that she didn't act the way the main character would typically act. She had a bit more power to her.
After the first book, most reviews about the series said that it got better. Lots of things changed after the first book or two, but I haven't found the energy to get past 'Crown of Midnight'. Once I started the second book, I was interested enough to get to the very end, but once again I've found myself in that state of disinterest.
I think it's very important for a book series to make sure the interest for the following books is maintained, so that's never a good sign. There are a lot of books to read, so making sure a book stands out is a good goal for an author. Then, after the first book comes out, the hype has to be maintained until the following year or two when the next is released.
That's not to say this series is not wildly popular enough to do without my interest. I still own four of the books in "Throne of Glass" regardless of whether or not I'll get around to reading them. That's also not to say I won't ever be in the mood to pick it back up again, either. Maybe once the sixth or so book comes out, I'll try it once more!