I know this is going to sound pathetic to almost everybody. But Optimus Prime had been done up as a hero character, and was one of the most believable ones presented to children by the media at the time. He had a lot of traits going for him -- intelligence, valor, honor, etc. He looked after his Autobots, led them into battle, rescued them from Megatron periodically, was kind to basically everybody. He was like a benevolent patriarch who showed up on television at the right time (I should mention also that this was during one of the steeper periods in the elevation of the American divorce rate) every day. So for him to die in the movie was a fairly wrenching thing to do to a bunch of kids; Optimus was the single most clearly likeable and desirable character, as well as the most prominent, in the whole Transformers thing.
In the comic books, Optimus was "killed" before the movie was released, in a different way (more treachery, of course), and his body was reprogrammed to attack Autobots. This lasted until the daring rescue effort led by Buster (the token young human boy, who played host to the Creation Matrix when Prime "died") brought him back. All that took about two monthly comic books to get through, and was pretty well done by the standards of that series.
Meanwhile, Unicron watches all this happen -- my g/f points out that the Virgin Megastore in London has a television-composite sphere display very similar to this one.
One other unrelated thing to mention -- in the comic series, the above-mentioned daring rescuse was conducted by the "Buster" human character with the assistance of "Jetfire." Jetfire was an unimaginately-named plane modelled marginally after the American F-14 interceptor, but the robot transformation strongly resembled the RoboTech model characters which were achieving a narrow popularity around the same time (you built them yourself from parts with glue, which obviously eliminated the majority of ten year-olds). It's possible that this represented an experiment as to whether the two proximate genres could be conmingled. If so, it apparently didn't prove economically profitable to those holding the reins.coalesce, emerge, fall