Like you I find rank and experience to be pointless and not a thing to obsess over. So some of this may have changed over the 6 years I have played and observed what you get experience for. But I believe most of it still applies (or hope it does). 
First off, there are two types of experience.
The first is Economic. You get points for each building you construct and then you get additional experience when you upgrade those buildings. I have never observed points for destroying or damaging buildings.
The second is Military. You get points for units built (iirc) and units killed. You get more points for higher level units than lower, as you would expect.
