NOTE: This post does not contain plot spoilers for the comics, but it does mention some of the characters that get more screen time than others and in which series.
During it's first season MTMTE shone the spotlight on many characters that don't usually get a lot of air time. Swerve, Tail Gate, Rewind, Skids; these guys have all come from relative obscurity (to general fans and especially within the IDW-verse) to fan favourite characters, thanks to pivotal roles and amazing writing by the MTMTE team.
I think this is fantastic (can't get enough of Tailgate & Skids) but I also think it highlights how the previous comic runs have given short shrift to the majority of even the 'A-list' cast.
Take Soundwave for instance. I've read pretty much the entire IDW TF run (except Hearts of Steel, but come on...) and I feel like I hardly know the guy. I know who he is, what he's like, etc, but the writers behave as if I already have the whole story on Soundwave. Aside from a couple of half hearted "oh, yeah, he did 'X' before the war" morsels we don't really know him except that yup, he's super dedicated to Megatron & the Cassettes.
Even in RID, I feel like I'm finally getting to know Starscream. Not just see someone who remembers how they were portrayed in a cartoon in the 80's. It's excellent.
Who else would you like to see become more developed as their own person, not just riding on what we're already supposed to know from other media?