These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Countdown to Final Crisis
Bile Fascination: A number of people have read this series and picked up the trade paperbacks because they'd heard a number of critics deride it (whether it be their peers or professionals).
Complete Monster: Superboy-Prime. Imagine Superman with his Silver-Age level powers and absolutely none of his maturityor moral boundaries. You could at least slightly sympathize with him in Infinite Crisis, but the first thing he does here is killing a pregnant woman, then killing that universes Justice League, and then blowing up their entire planet, and then destroying countless other alternate earths during his travels, purely out of spite because they're inferior to his "real" earth. It gets far worse from there.
The whole series can be this if you take the view that it's so bad it's funny. It's all unintentional, but there are some comedy gems like Jason Todd (the gun-wielding crazy guy) telling a Monitor that he's not fond of gun-wielding crazies, Donna Troy's "I'm Donna Troy BITCH!" and of course "I'LL KILL YOU TO DEATH!" from Superman-Prime. There's also Jimmy's awful Mr. Action costume and Darkseid on Mary Marvel's couch...
It Gets Better: The Earth-51 arc. It went on (almost) uninterrupted by unrelated things, had a lot of cool fights, some really strong moments (like Jason Todd meeting its Batman and this Batman's death) and ended with a bang. Sadly, right after that, the series became much worse than ever before.
They Wasted a Perfectly Good Plot: Jimmy Olsen gets all his Silver Age powers back! Brother Eye merges with Apokolips! Mary Marvel, "deserted" by Captain Marvel, turns to Black Adam! Harley Quinn and Holly Robinson vs. Granny Goodness! A crazy, universe-hopping adventure searching for a character missing for (relative to the reader) years! Paul Dini writing! A lead-up to the biggest Crisis of all! A followup to the beloved 52! A look at the seedy underbelly of the DC universe from the perspective of two repentant Flash villains! The genesis of the Kamandi-verse! What Could Possibly Go Wrong? Short answer: EVERYTHING.
And a lot of these perfectly good plots were really good when Grant Morrison pitched them for Final Crisis. They were so good in fact that DC decided to shoot their load early and jam them into this, the lead-in to Final Crisis. Which was only the beginning of what went wrong.