In the first cartoon, Megatron, wasn't just fun/charismatic/entertaining - he was considered scary. Why? He was a threat, harming 'bots & humans alike. Among 80's US cartoon villains, Meg's was THE credible bad guy on kids TV. I know of only 2 that rival. This left an impression on us kids back then.
I can understand, 100%, why someone who looks back at G1 Megatron in the Sunbow/Marvel Studios production, & ask, what's so special? Don't forget the context of the character in the 80's on kids TV.
In fact, he was perceived as TOO scary, and after the TF the Movie which saw Galvatron, who was intensely scary for kids back then, they deliberately chose to make him, so inept, that he was not scary at all.

One might argue that Megatron is a classic hero character, translation, not being a good/moral hero. Mythology is filled with "heroes", however, most weren't good like we would think of a hero today.
If you look in Marvel comics in the US/UK, Megatron was truly awesome, & all the Galvatrons, [plural, no -'s, not possessive], who had more than one incarnation, Galvy was even more dangerous and threatening than Meg's.

I believe that DW writers understood this. They knew why G1 Meg's/Galvy left such an impression G1/G2 fans. Megatron "the Great", [like ancients viewed Alexander - great not being a positive], really started to hint his gravity, by Book 3 of TF G1, & the Armada finale. The black speech boxes, & G1 Galvatron invasion with other classics?
Given who the DW writers were in the 2000's, it's no surprise that Meg's was heading towards a super event, & his implied role. Many of these incredible writers are still involved, as they were in the 80's.