Remember Marvel's What If? series? Each issue would take a familiar element from somewhere in Marvel's (usually recent) past, and put a spin on it. Alternate history of a fictional universe. That's exactly what Infinities was. Originally, this was published as three separate miniseries, one for each episode in the original trilogy. Each miniseries was entirely separate, so this isn't one long alternate universe, but three shorter ones.
So what changes? In A New Hope, Luke fails to destroy the Death Star. This leads to Leia being captured and turned to the Dark Side, Luke spending several years training with Yoda, and Yoda ending the series with one hell of a bang. In Empire Strikes Back, Luke dies on Dagobah and Leia becomes a Jedi in his place. And in Return of the Jedi, Threepio is accidentally destroyed in Jabba's palace. I know that sounds minor compared to the previous series, but it's more like a row of dominoes being tipped over than one big continuity bomb.
And honestly, it's all kinds of fun to read. Sure, the apparent need to force all of the scenarios into the best possible cases is a little annoying. Let all hell break loose! Still, the characters act and mostly sound like themselves, and the art isn't bad. Fun, but it doesn't add anything to the Extended Universe, and it isn't intended to.