It isn't just their worst album, but also the worst album made by any established band, ever.
I think the main reason I hate this album so much (and why so many other people do too) is because of the fact that it had been 6 years or so since their previous album and fans were really excited about the hype and how it was rumored they were “returning to their roots” in this album.
As a huge fan of "Kill em All," "Ride the Lightning," and "Master of Puppets" who was actually a bit disappointed with the black album's new sound, I was very excited upon hearing this news.
However, "returning to your roots" apparently means using equipment that you bought with your paper route money and recording with one microphone hanging down the middle of your Mom's garage and also letting your producer be part of the creation process and even play all the bass on the album.
There are apologists out there that say “you just have to pretend you aren't listening to Metallica and imagine it is any other band, then it is pretty good.”
What kind of crap statement is that? People wanted to specifically listen to Metallica, not just “some band.” I remember the saint Anger I felt after having spent $15 on this piece of crap that I should have turned into an AOL drink coaster after the first play-through. 
It was 2003 and back then people didn't put their entire lives on the internet (Facebook didn't exist yet) but I was so St Angry about this purchase that I found every available forum to rip this album apart to anyone that would listen. However, when I did find those forums there was already twenty thousand people there saying the exact same thing. Many fans were saying they would never buy another Metallica album ever again – which is great timing because right around 2003 is when people were no longer buying CD's anymore and Metallica's Lars Ulrich became a spokesperson for the anti-piracy witch-hunt that eventually failed and is subsequently why we now have Spotify.
I hesitate to even include a video because no one should be subjected to this drivel but it seems to be what I do so here is one of the songs that was considered to be good enough (by the production team) to make a video for and also feature as the first song (or hook) on the album.
The lyrics are trite, the drums just sound terrible, and for me and millions of others it seemed as though Metallica was just going for a money grab and because of their star power they knew they would get away with it. They did get away with it too! This P.O.S. album reached number one in sales in many countries only to see it quickly dive bomb off the charts entirely after just a few weeks.
I was at one of their concerts and when James Hetfield said something like “anyone wanna hear something off our new album?” The crowd booed and the band laughed to one another off microphone, like they knew what they had done, as though they actually had disdain for their fans and knew they could do whatever they want. Thankfully, they only played one song from their new album.... even they realize that trying to promote it was impossible.
What do you think? Convince me I am wrong if you want. I think saying that this album has any merits is a pretty indefensible position to take...