Cute topic.
There are several things that have "ruined" Transformers.
Not financially, but either for me personally, or, imo, have hurt the brand's uniqueness.
I'm a continuity fan.
It was one of the things that made it stand out from other all the other dull mech shows, so the addition of new continuities still let me enjoy it, but only in a What if? kinda way, where I don't actually care what happens to these people.
I also hate things stuck in the past, and don't consider redoing G1 over and over is actually giving us something new, so with each new reboot I think it looses a bit of it's magic, and I lose a little more passion.
The second was the Beast style. I didn't like it personally, but in Beast Wars at least it made sense as they had moved on tech wise; they weren't Transformers, they were something genuinely new.
When I see them in later lines though, they just seem out of place among the more traditional looking bots.
Well, with the exception of Animated.
The third is the movie.
I was a G1 comic fan and always thought the show was bad, but the movie makes it look like an intellectual masterpiece.
When I watched the movie, I could see every negative thing everyone has levelled against the franchise over the years, it made me hate Transformers.
And the designs, whilst not all bad, lost too much of the visual identity imo.
Whatever people think of "traditional" style (Diaclone meets Microman) it was different to everything else out there bar the lines it came from.
Macross and Gundam have moved on whilst being very traditional, but Transformers hasn't.
Not ruined forever as some fans claim, but certainly weaker for having done it, imo.