The big problem was Boromir usurping his brother Faramir's call. Both Faramir and Boromir were called in dreams (Boromir once, Faramir several times) to go to Imaldris and seek Isildur's Bane. Had Faramir gone, things might have proceeded differently for the Fellowship (not that it didn't work out all right in the end).
If Faramir had gone, then from a writer's point of view, Tolkien would have to find some other catalyst to get Frodo to leave.
Of course, I'm oversimplifying the plot. Tolkien (as genius as he was) could have given Frodo a million different reasons to leave...
But, through the character of Boromir so much is linked. Gondor, Aragorn, the White City, Boromir's frustration of being the one always having to defend Middle Earth (with only Rohan to help). Again, that narrow mindedness on Boromir's part that only Gondor defended Middle Earth just wasn't true. But to Boromir it was. That's why he wanted the ring. Gondor was failing, was falling... and there wasn't enough men left to defend it. So, he needed something stronger than the enemy to defeat the enemy. The ring was perfect. It was his only hope.