I agree about the untapped potential of Dark Avengers, started out with so much opportunity but really began to taper off very quickly, hate when that happens! x)
Probably the most underutilized potential for me was on DC's Red Lanterns, I didn't know what to expect with the series but was very back and forth on the storyline. Particularly when Supergirl became a Red Lantern and they made a huge deal about how much more powerful she'd become... and then they totally brushed her off after about two issues, with her barely having any impact in the story or universe at all. I dropped it from my pull list shortly after that! lol
I've actually never read the Extremis run, but heard a lot of great things about it - I will absolutely check it out now, thanks for reminding me of something cool I should've picked up already!