Lyrically, I'd say Het went downhill hard on St. Anger (though to be fair, some of that, like "MY LIFESTYLE DETERMINES MY DEATHSTYLE," can be blamed on Kirk or Lars). The decline to there from the Loads is actually rather shocking, in my opinion. The Loads have some of my favorite Metallica lyrics, and SA is just not good in that department. From SA to DM he definitely took somewhat of a step up, though I don't think he nearly reached back up to the level of lyrical quality on RTL-RL.
At least with DM, though, the lyrics are pretty inoffensive. A majority of the songs are just generic metal imagery that is neither notably good nor notably bad. Some of them seem personal, yes, but it's hard to get much out of them. Usually you can tell what he's singing about but don't really get anything too compelling out of it. At best you get a clever turn of phrase ("Like a poison / That I swallow / But I want the world to die") and at worst you get a line that sounds like it means nothing ("Reaper... butchery / Karma amputee") or is a bit cheesy ("love is a four letter word").
I do think The Unforgiven III has some nice lyrics, but not as good as the first two.
I do worry that he's going to take whatever lack of inspiration he has lyrically and just go for gratuitous violent imagery or profanity for the sake of being "metal." I really don't like that at all, and it does seem like a direction that is possible based on Hardwired and some of the song titles ("Murder One"? Yikes).