In places, the graphics are amazing, superior to the original, especially the sea/water. The voice-actors are as good as they were, perhaps even better, than in the original. So why the 3 stars? It's the plot, there really isn't one.
In the original Ice Age the story was about a group of animals reuniting a human baby with the tribe that lost him whilst set against the back drop of an encroaching Ice Age and all the melodrama that's entailed in that. In this movie, the ice is melting and the animals have to get to the other side of the valley, but other than that there isn't really any other constant story to hold the movie together, unlike the original. Instead, what's offered is an excuse for an animation sequel that falls short of the original by quite a margin.
As already mentioned, the visuals and voicing are high quality, and there are even sufficient humorous episodes to put a smile on your face so that you don't finish up thinking you've just wasted and hour and a half, but this is more like a soap-opera rather than proper film. In this offering it's the scriptwriters that are the let down.
The movie is peppered with well-rendered yet ultimately simple filler animation sequences, a couple of which are more reminiscent of early Disney animations, which is a bit of a cop-out these days. This movie desperately yearns a decent storyline to carry it (sorry, but the back drop of melting ice just isn't enough).
This animation baulks the trend over recent years for presenting mature humour in a child-like packing and instead goes just for the child-like packing (which it achieves admirably). But... it's not cleverly scripted, like The Incredibles, Shrek, Monsters Inc, Antz, Toy Story or even the original Ice Age.
I'm sure it will be delightful for younger children and anyone else with a young-at-heart-and-head disposition.
I'd give this movie an overall 67% entertainment rating.