I think you can not put it clearer than tedster did: just do the basic SEO stuff and make sure the site is spiderable and contains your keywords in the right places - Then go for user engagement.
Also: a new site of mine is spidered well, but the new pages are NOT showing in the index. Now 1 week after the first 700 pages are taken - still no sign of them in the index... Google will IMHO not do ANYTHING until Christmas.
My first SEO run was in late 2001 - since then the index of G changed ALWAYS substantially in January! Like a ping pong game: big updates in fall, big updates in January.
Just 2 pennies