A Good Start, But I'm Left a Little Wanting
Crossovers have a set formula, as any other works of fiction; you can't have all the action up-front, because it will either wear our your readers of leave you with so little gas in the tank that by the end, your story will be spluttering.
Fraction is quick to throw us into the thick of the action with a riot and storming-a-nazi-fortress sequence, then brings us back down with interspersed scenes that are a bit low-key. This is great because it keeps us on the edge of our seat, knowing that a conflict is going to happen sooner or later. Sowing seeds of doubt between a number of the characters will likely come back later when this "fear" concept gets explored further; as readers, we know a lot more than the characters, and it contributes to a growing sense of dread as you realize what's coming.
Though Marvel has claimed the contrary, you're going to be left confused with this issue unless you're pretty current on Captain America, Invincible Iron Man and Thor. A lot of the concepts that are being fleshed out (the reconstruction of Asgard, Stark Resilient, Sin) are dependent on that prior knowledge; though Fraction does a good job of giving quick descriptions, this isn't enough to get the full effect of the story he's trying to put across. Give yourself a quick refresher in our wiki before you give it a read.
Fear Itself 1 is hard to grade, as it's the first issue of a crossover. It's setting up a lot of things that may be boring, but they're important to the successful implementation of a complicated story arc. The villains are threatening, the heroes know something evil's on the horizon, and we're just going to have to wait until issue two to see where it all leads.
For those who saw my ComicVine Crossover Bingo post yesterday, here's an updated card after issue one: