I COMPLETELY agree with you on Air Gear! It was SUCH a bad ending. I had watched the anime series before reading the manga and it was so screwed up that I was left thinking "There's got to be a manga for this...or else I am screwed and that is the most awkward series ever." Thank goodness after looking it up on Wikipedia I found out there was indeed a manga and started from the beginning again. I can't believe they left it off only part way through the story. I'm assuming it's because they were going to do multiple seasons and lost the funding for it, but still...
Another really bad ending to a GREAT series (imo) was Kaze no Stigma.
***Spoiler!***
It ends with Ayano saving Kazuma and turning him back to normal and they save the city from destruction. Except Bernhardt and Lapis get away and are never defeated and the relationship between Ayano and Kazuma is never really revisited. The series and story just... abruptly ends. So much more could have been done!