The original Ice Age film was one of the first I ever actually owned. It was one of the best films I had seen. When the second film then came out, I loved that one too. The films of Ice Age have references for all ages, and when I was growing up, I might not have understood them all at first, but over time I did.
I have watched all four films countless times [the fourth less so, due to it being newer], and each have held a lot of thoughts, and all of them are different depending on which film it is.
The first will always be a classic and heartwarming tale in my eyes. From first seeing the meeting of Manny and Sid, and what they mean for each other, and then when Diego arrives to collect the baby from them, the character interactions are great, and throughout the film we see deep characterisation of Manny, Diego, and even Sid [though not so much]. The story as a whole is crafted with moments like this, from beginning to end.
Diego's story is especially strong, as we see him go from merciless hunter of a pack to saviour of a friend and member of the herd. Manny saving Diego from falling at great risk to his life made a change in Diego that at first he doesn't recognise, and throughout the film, this is apparent in how he acts. He finally comes to the change of heart when Manny is in danger that could get him killed, and since Diego set them up near the start of the film he feels it his duty to save Manny.
When the baby is returned to the humans who the tigers chased it from, the feeling of a job well done and loss at what was gained and lost through the journey is clear in that scene. Manny might not have overcome his past, but he at least recognises it.
The second film then, continues the characterisation of Manny's past, and throws in a problem to face - that of another mammoth who thinks she is a possum. Ice Age 2 introduces us to Ellie, Crash, and Eddie. The latter two being the possums who Ellie was raised by and with during most of her life. The flashback scene we see this in has the right emotion to it, enabling us to connect with the predicament that presented her with what she thought she was.
Manny's past conflicts with his feelings for Ellie, and before they meet, Manny is constantly questioning whether he really is the last mammoth left. The excitement shows clear as day when he thinks he hears another mammoth, and the disappointment upon it not being a mammoth shows equally clear, so much so that he doesn't even want to face Sid and Diego and requests some alone-time.
Diego again faces characterisation as Sid teaches him the meaning of fear and how to face it. That seems to be Sid's role within the films. He has more brain than he lets on, and with the way he acts, you wouldn't really think he is a philosopher. Or... maybe that's too big a thing to place on him, but he seems to be the teacher of lessons within these films.
Ice Age films don't really have massive complex plots, but more than make up for that with the amount of characterisation they put the characters through. The third and fourth films show this greatly, with the third film being about rescuing Sid from dinosaurs that have been living under the ice for thousands of years, and the fourth about Manny, Sid and Diego trying to get back to the mainland after the world splits apart, and Ellie and their daughter learning the true meaning of friendship and love while the mainland animals try to find safe refuge from the ever expanding rock face that is tearing up the mainland.
While the first two films had reasonable stories though, making them more understandable in the situations, the other two films go beyond reasonable and instead places the characters and situations in non-recognisable, or at least less understandable, locations. This brings the characterisation to the fore a lot more, I feel, as the main story won't hold your attention. Making a film based mostly off characterisation is something I like, and most animated films focus on this. While I won't go into too much detail, the likes of Cars and Madagascar feature a lot of characterisation more than they do story plots.
I might have gone a bit off board toward the end, but after watching the first two films again, and watching the next two over the next day or so, I am loving watching them and getting to know the characters. If I had to choose from any of the films though, it would still be the first - less for the nostalgic quality of being the first film I ever owned, and more for the perfect blend of story and characterisation.