Story: Oban is actually a pretty archetypical story about a girl trying to find her family, racing, love triangle, blah blah blah. While it is a pretty common basic line, the creators did a fantastic job mixing everything up and ending up with a coming of age story with sci-fi elements, romance, adventure, and lots of interesting turns.
I'll be the first to admit that it seemed to go on for a while, but it was overall incredibly enjoyable. Eva/Molly is a bit of a brat sometimes, but I couldn't help but feel for her as she realized her father didn't even recognize her, even if it had been ten years.
And then the plot twists! In the interest of trying to be spoiler free, I won't go too much into it, but the story kept me guessing but still didn't go too far out of the realm of possibility, and picked up threads I'd thought were long forgotten in time for the big finale. At the end, I really had to fight back the tears.
Animation/Art: Oban did really well in a traditionally taboo field of mixing 3D with 2D animation, something I generally detest. The 3D models were set in 2D backdrops, their toon/cell-shaded style almost perfectly matching the animation style and used in a consistent (but moderate) manner.
While the style itself is really simple, they pulled it off very well and managed to make everything incredibly expressive with so few details. I am also very impressed with the character designs and backgrounds.
Sound: The music is pretty good, although it can get a bit repetitive and in some places I thought should have added more music, but the ending and opening themes fit so well (from what I could understand) with the show that I didn't skip them as I normally do when watching an anime. I'm seriously considering buying the soundtrack myself.
Characters: Again, pretty archetypical: The best friend mechanics- one geeky and the other pretty macho, the super manly samurai-esque star racer, the controlling overbearing manager/father, the clueless irrational cuddly soldier, the gentlemanly prince, the charming spunky girl hero... However, the characters were fleshed out very well, even the side characters with little to no dialogue I found myself wanting to know and hoping they would survive.
Overall, I suppose that many people probably wouldn't like it, but I found this to be a highly enjoyable series, with characters I loved and a story and designs and art that I just can't find words to describe just how amazing they are in my opinion.