The fakenime look of it. Not that it will matter if the story is great in which case I'm sure I will get over it eventually.
I totally get where you're coming from. Before I watched the show, I felt the same way. However, while at first glance it may seem like the product of the cynical anime-pandering that animation studios and comic publishers were doing in the early 00s, Avatar actually does a remarkable job of setting itself apart visually from all of the other "Americanime"-style media of that era. It has its own strong, cohesive art direction that exists independently from the fakenime look. It's not really the type of art direction that you can appreciate from screenshots and clips alone; rather, you kind of soak it in as you move through the series and see it all come together.
In the artbook for the show, the lead art-guy/co-showrunner (his official title escapes me), Bryan Konietzko, is very much aware of the fact that he is an "outsider" when it comes to the anime/manga art-style, and at one point, he explicitly states that he's not very good at emulating it. And honestly, if you look at shots from the series, that's not exactly hard to see. It's clear that the safest and most inoffensive (maybe even boring) style was chosen for the series. But as you watch Avatar and see the art style in context, you kind of realize that that's the point; the plainness and relative nondescriptness of the style allows the art direction to take center stage. Avatar doesn't expect to get anywhere on the strength of its art style, nor does it attempt to (though that's not to say it should be completely forgiven for looking generic). The fact that it looks like anime is almost irrelevant. It doesn't try to *be* anime; it doesn't try to *be* Japanese; it doesn't fetishize this art style in order to appeal to some rapidly-growing contingent of consumers. It simply uses it to convey the art direction; it uses it as a means to an end.
(or maybe I'm just full of shit and have Stockholm-Syndromed myself into apologizing for Avatar's art. lol)
Though seriously, I think you'll manage to look past it or come to like it. I was a gigantic weeaboo when I first watched Avatar and came in wanting to hate it so much because it was American, but it ended up stealing my heart. Hopefully you'll have a similar experience.