Well, remember what the reaper said to Shepard as he was running from the exploding Collector base (oops...sorry for the spoiler):
"Human, you've changed nothing. Your species has gained the attention of those infinitely your greater. That which you know as Reapers are your salvation through destruction."
So apparently the Reapers are now looking at humanity as a significant threat. I would think the attack on Earth is going to be pivotal but it won't be the only important thing going on by any stretch of the imagination.
Bioware has stated that decisions made in ME1, which had no or seemingly insignificant consequence in ME2, will have noticeable effect in ME3. I think it may be safe to assume that things like sparing/destroying the Rachni queen, whether or not you kill Wrex on Virmire, killing/reclaiming the heretic Geth, encouraging/discouraging the Quarians in retaking their homeworld, whether or not you tell the Illusive Man to go bleep himself, etc. could lead to major story lines in Shepard's attempt to gather allies to fight the Reapers. Shepard may be the ultimate badass in the galaxy, but he can't fight thousands of Reapers with just the Normandy and a dozen specialists.. he's going to need an armada, and that means Rachni, Krogans, Quarians, Geth, the Council fleet, Cerberus, the Alliance, and anyone else he can convince.
It's going to be epic. I fear not.