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.
Adaptation Displacement: Inspired by the book The World Without Us which, due to its success and also to perhaps because it was named one of Time Magazine's Books of the Year for 2007, suddenly made this subject very popular. The National Geographic Channel also had a very similar show at the same time called Aftermath: Population Zero.
Inferred Holocaust: Not just the humans, but the animals that are bound to die in our absence.
Nausea Fuel: "Last Supper" (Season 2, Episode 4) features a Fort Worth, Texas supermarket that was abandoned for several months in 1999 with all the food still inside it, and goes into great detail about what happens when you leave an entire grocery store's worth of food sitting unattended in the summertime in Texas.note After complaints from neighbors over the smell, the supermarket was fumigated and stripped to the bare walls.
Nightmare Fuel: Abandoned supermarkets. Especially the meat and deli departments. See above.
The thought that our precious friends will become vicious hunters. The notion that animals will forget you and humanity in due time. The fact that the smaller and weaker pets, such as toy breeds, will survive along with some of the more poorly designed breeds.
The ones that don't starve to death in our homes and zoos since we aren't around to either feed or free them, anyway.
How about the fact that eventually, given millions of years, there will be no trace of evidence that we humans were ever here, save for perhaps some fossils if we get so lucky. All of our achievements, all of our buildings, all of our records now lost and buried with time. Like we never existed in the first place.
Tear Jerker: For pet lovers, at least. The pilot episode of the first month or so takes extra care to mention the millions of cats, dogs, hamsters, and fish all locked inside their houses, waiting for humans that will never show up. And then they had to show cats pawing at the doorknob and dogs scratching at loaves of bread, desperate for food.
Also the Real Life fate of the animals in the New Orleans aquarium, nearly all of which died after Katrina because the facility was evacuated and nobody was there to keep their tanks' water from turning foul with wastes.
They Just Didn't Care: One episode goes on about how cats are natural born predators, and will thrive in a world without people because they'll revert to a predator existence. This same episode claims that dogs are screwed because they're too stupid and sheltered to survive. Miniature poodles and toy dogs, maybe, but anyone who owns a working or hunting breed knows that these dogs are highly intelligent and frequently capable of taking care of themselves if need be.
Even some of the smaller dogs could survive, filling the ecological niche once held by foxes and weasels. Dachshunds in particular are very intelligent, highly adaptable, and extremely aggressive at need (they were originally bred tohunt and kill badgers in their tunnels!)
To be fair, they did praise Lacy Dogs and German Shepherds (although they were fixed).