If you remember, Khal Drogo wasn't all that into the idea of conquering Westeros. Viserys was the only person that was hot to trot to do it. And by the time of the botched assassination attempt, he'd already received his "crown of gold."

It was the botched assassination attempt that really put the battery in Drogo's back. Otherwise, they might have wandered Essos for years. Ned was right that they weren't a threat until they could actually cross the sea.

Also, if they hadn't tried to kill Dany, the Dragons many never have been reborn. Drogo going on a rampage leads to his death, which leads to the rebirth of the dragons, and so on and so forth.

>also what ever happened to that one guy from S1 who Varys was
>talking to? He was kind of the go-between who gave Dany the
>dragon eggs? Dude seemed crucial and just disappeared

Ilyrio Mopatis? Yeah, like a lot of the characters, they just kinda stopped caring about him. Hate to be that guy, but he's a lot more important in the books. A lot of the stuff in Season 5 involving Varys was Ilyrio in the books. It appears they eventually decided to combine the characters (they share a lot of history).