I'd say a lot of concepts are revolution vs. evolution.
1980's for instance- Let's have robots than transform into vehicles?
1990's- Let's have robots, transform into vehicles, combine with other robots, and shout out attacks so loud that people's ears will ring! Oh yeah, a mere gun isn't going to be epic enough, let's give him a giant hammer of death!
1980's- Cute animals! Just one kind though.
1990's- A ton of cute animals! Two of every species! And we have tough looking ones too!