My favorite part of DragonBall Z, which is also the part where I started watching the series, was the time travel plotline. It was pretty exciting. A character named Trunks, who nobody had ever seen before, mysteriously appeared and killed the series' main villain, without even breaking a sweat. After that introduction, he turns to Vegeta and says, "Hi, I'm your son from the future, who hasn't been born yet. I went back in time to prevent the apocalypse."
Okay, so it didn't happen quite like that, but it was still pretty cool.
The time travel material was very well done. Instead of having one timeline, like Back to the Future, there were multiple timelines. DBZ kept things relatively simple, by only having two timelines: the main one where everything happened, and the horrible future timeline that Trunks was from.
Here's the part I don't like about the time travel in DBZ. Fans of the show claim that there has to be a third timeline, in order for the first two timelines to exist. This timeline is seen briefly, in a filler episode. There also has to be a fourth timeline, in order for the third timeline to exist.
The show itself doesn't go into detail about Timelines 3 and 4; this is mostly fan conjecture. Timeline 3 is the Worst Timeline Ever, in which everyone dies. The only survivor is the main villain who steals a time machine so he can kill everyone in another timeline. In Timeline 4, Trunks is sort of boring, and he stops the bad guys with a remote control instead of fighting them.
The points I'm making:
1. The two timelines that are featured in the show are good. I like them.
2. The two timelines that are not featured in the show are bad. I don't like them.