There's nothing that screams to me "must get immediately". Time Machine looks pretty cool, but I'm a little nervous about my co-workers abusing that feature to the point of frustration (for me, that is). IMO, Spaces is something that should have come with the first edition OS X.
While I am sure there are some bells and whistles worth having eventually, I think waiting few months isn't a bad idea. No matter what, it can't be worse than that other operating system.
And for what its worth, the new version of Ubuntu is pretty cool. I'm actually more excited about that upgrade.
I recall there being a thread devoted to a rather simple opt-out option for pop unders. It's something that I ran on a previous computer, but that computer is now spare parts, and I'd love to run it on this one.