Actually, I'd have to disagree about the "world's" being different; there are many points of continuity, and the characters go through changes over the course of 12 movies and the tv series. The strongest link is shown in movie two, with many "bridges" to the first movie being apparant:
1. Cera's father is still a speciest/racist, as his lecture to Cera was "never play with longnecks, spiketails, and bigmouths ("swimmers"/hadeosaurs)".
2. We still see that similar tensions still exist between the characters, especially between Littlefoot and Cera.
3. The threat of death and/or injury is still hanging over the characters in a way it hasn't been in the recent sequels. Some examples include the volcanic geysers that Littlefoot falls on and is nearly scalded by, the nighttime chase scene, the point where the Struthiomimuses were attacking the characters, the attack of Chomper's parents, the active volcano, and the point where Littlefoot was nearly dropped off of a mountain. In the most recent movie, there were only two points of danger; when Guido was sleepwalking (to be fair, that had many dangerous sub-points in it), and when Tricia was going to go over a waterfall.
4. Racial/species issues are brought to the forefront. Although we haven't seen such issues since the fifth movie, we are seeing them again in the tv series.
Also, Littlefoot's character itself is still the same; the same issues from the first movie are still with him 11 movies and a tv series later.