Before getting into manga, I was a pretty big X-Men fan. I eventually got frustrated and gave that up thanks to countless plot holes from constantly changing writers and having to buy 7 or 8 different books a month to keep up with the continuity. Like a couple others mentioned, it was the whole Onslaught fiasco that finally drove me over the edge.
However, there are still some American series I love. First and foremost is Transmetropolitan, followed closely by Watchmen and Preacher. All three of which are over at this point, but I still reread them constantly. I've also been getting a bit into Marvel's Ultimate line (particularly Ultimate Spidey and a it of Ultimate X-Men), as, for the moment, it lacks most of the things that caused me to seek other sources of entertainment years ago.