In the fourth movie, when Diego remarks dryly to Granny that he's "glad to see her," it's played for laughs. Upon second thought, however, she just nearly poked his eye out. Of course he's glad to see her! He's glad to see anything.
Roshan's name means "Light at Dawn" in Hindi and it could be argued that he leads the three main characters out of their dark pasts and is a major factor of them becoming True Companions.
If you pay attention, it seems that most of the movies contain a reference to the next movie in the series.
At the end of the original Ice Age Sid says something about global warming. Cue Ice Age: The Meltdown.
In Ice Age: The Meltdown, when Manny says mammoths can't go extinct because they're the biggest things on earth, someone counters “well what about the dinosaurs?” Cue Ice Age Dawn of the Dinosaurs.
Ellie takes a quick little jab at Manny in Dawn Of the Dinosaurs about not being able to relax about the baby, and wonders "how he'd deal with the teenage years". Cue the next movie, where she's a teenager and Manny is not dealing with it in the best way.
Some may wonder how it is that Peaches seems to pull so many more stunts than her mom Ellie. Obvious answer is that she’s younger and lighter, but the other reason is that Ellie was in a unique situation and had to figure out how to manage Possum stuff with a Mammoth body all on her own. Peaches had her mom to teach her how to do the basics, so she could come up with some tricks of her own.
Manny displays serious overprotective tendencies toward Ellie and Peaches in particular. It's understandable, considering what happened to his last family.
In the Original, knowing that Half-Peak is confirmed as being a volcano (a dormant volcano) in the novelisations, it makes perfect sense where there's a lava flow underneath the ice. The lava flow laid dormant there after the last time the volcano erupted!
A lot of Manny's lines from the first act of the first movie make a lot more sense once you know his backstory.
Manny (to the rhinos): I don't like animals that kill for pleasure.
Diego's body isn't bloodied for one simple reason: most of his injuries are internal (and most of them can be attributed to when Soto struck him into the boulder, no doubt causing a concussion and myriads of broken bones scattered throughout his body) and it's possible that where Soto gashed him he deliberately laid on top of those wounds so that the other members of the herd wouldn't be unduly worried.
Diego's attempt at turning a tiger pawprint into a human footprint in the first film was laughably bad, but probably made sense from his standpoint. He drew a bare foot rather than a shoeprint because none of the animals completely understand when or why humans wear clothes, and he gave it exactly four toes because that's the amount of digits that most animals (including saber-tooths) have in the film.
In the second movie, Manny talks about how large animals like mammoths are too tough to go extinct. A mother Gastornis counters by bringing up the dinosaurs. Gastornis is a type of bird. What better animal to mention the dinosaurs in such a discussion...than an actual dinosaur?
Instances of Sid being a Jerk Ass to Manny and Diego could actually be him biting back at them for them being similarly mean to him.
Sid's family abandons him, with no idea that he'll survive the week. Knowing Sid, it's very possible that they suspected that he wouldn't survive, and they left him anyway. And then, they did the same to Granny, all because they thought the two were "screw-ups." No wonder Sid is so desperate for affection.
In the fourth movie, we see lots of teenage mammoths, but Ellie and Manny seem to be the only adults. What happened? Did all the other adult mammoths all die?
Did you even watch Meltdown?
That in no way addresses this concern. The point is that clearly, there have been some other mammoths living in the same area as the group, and yet we only see the teenage ones. Meltdown was two films, and clearly a decent period of time in-universe, ago. Something appears to have happened since, to the mammoths living in that area at least.
What is Diego eating throughout the movies? It's established that all the animals are sentient, and he's friends with quite a few of them. But, as a carnivore, he has to eat something.
Speaking of which, what's Shira going to eat?!
In the 3rd movie he's shown hunting a deer (but not succeeding) so it seems that they would still hunt normally, as long as it's not someone they know personally. Also fish is always an option.
It seems like only mammals and birds are sentient.
In the first film, Manny was heading towards the freezing wasteland and it's implied that this wasn't too long after his family was killed. Was he trying to commit suicide?
Remembering that Manny's family were killed recently makes a lot of the film's scenes much darker. Particularly the scene where he hands Roshan to his father. The father sticks a spear (the same weapon used to kill Manny's family) right in Manny's face, and Manny shoots him such a stony Death Glare that you have to wonder whether he was physically holding himself back from maiming the guy with it out of mindless revenge.
In the cave painting scene, note that Manny's mate never stood a chance. Surrounded by speared humans, she could only shelter their calf helplessly. This makes perfect sense when you realize she had no tusks.
The official novelisation has Soto point out to Diego that Half-Peak is a volcano before instructing him to bring back the baby or Soto would kill Diego himself as retribution. Later on in the film, Manny and Diego and Sid nearly escape with their lives after the dormant lava flow underneath Half-Peak erupts. It makes one wonder if Soto knew it was going to erupt when he went on before Diego, thus dooming him....
See Fridge Brillance on this page.
Also, Soto has a victory or death mentality. Earlier in the first movie Soto tells Diego he'd better bring the baby alive or be "...willing to serve (himself) as a replacement."
If you pay attention then you’ll notice that there has been an on screen death in every movie so far. In the first it was Soto being crushed by icicles and Nadia drowning and the scene with Diego dying since the director commentary said it was a real death, in the second it was Fast Tony’s unfortunate glyptodont friend Stu (and later on, the killers themselves), in the third the bird thing that Sid threw off a cliff thinking it could fly, and Captain Gutt eaten by a siren and many of his minions falling to their deaths in the fourth.
While it was Played for Laughs, watching what was presumably the entire dodo race go extinct on-screen was a little heavy for a children's film.
The fourth film begins with Sid's family arriving in a log sled and tricking him into keeping care of his Grandmother before abandoning him again. Then the Continent begins to crack apart, and Sid's family is never seen again.
Buck and the dinosaurs were also beneath that continent.