Boredom, sexual frustration, dissatisfaction with life in general...
Or maybe it's just natural that people will have opinions that differ from your own. I have no idea. In any case, if you think this is bad, just wait until the album has been out for a while. Then the bitching really begins.
There was bitching for ATYCLB. Sure, the new U2 aspect was fun at first. Then people started complaining about how boring and safe the album was. "Pop" was praised for being so adventurous.
Of course, in '97, WIRE was around and while "Pop" had its moments, there was negativity there too. People complained about the odd sounds at the beginning of "Gone", how "Discotheque" had horrible lyrics, how "Staring..." was a Beatles' rip-off, how "Mofo" and "Do You Feel..." (along with "Discotheque") were bad copies of Prodigy and Chemical Bros., how "Miami" was ruined by the screaming, how boring "Velvet Dress" was, etc. But as the album didn't sell that well and at times the tour struggled a bit in the U.S., it seems U2 fans rallied around U2 and over-praised the album. In other words, U2 fans didn't have to bitch - the world did the bitching for them.
Then HTDAAB came out, and slowly ATYCLB started getting praised. Now ATYCLB is praised as this God-like album - compared to the horrific evil of HTDAAB.
So my guess is that after the excitement of NLOTH wears down, HTDAAB will slowly get more love, while NLOTH is torn apart. And on it goes.
Of course, if NLOTH is torn apart by the press, as "Pop" was, then I guess U2 fans will rally around that too (with some dissenters stating how NLOTH is crap and U2 deserve to be mocked).
And perhaps in 5-10 years, people will praise NLOTH as a great album.
I've played on U2 sites since 1994/1995 and in all that time, U2 fans have had one thing in common - the love of just tearing down the band. U2 have a hit - that's bad. U2 don't have a hit - that's bad. U2 are serious - that's bad. U2 have fun - that's bad. There were people who hated JT, but people who hated ZOO TV. People who wanted experimentation, but not what U2 offered on "Zooropa". People who wanted more JT, but couldn't stand the thought of U2 repeating themselves. People who wanted more Edge vocals, but didn't want Edge to be a lead vocalist. People who didn't want the echo effect, but didn't want Edge to be like all the other guitarists. People who wanted more Bono, but wanted Bono to shut up. People who got tired of Bono's wails, yet complained when he sang with a gravelly voice.
Bottom line: U2 can't win with their fans. Except for one area - $$. With 166M albums sold worldwide and huge hit tours, it's clear that despite all the bitching, U2 fans show their devotion with their wallets, not their mouths.