+1 on the onion - but also, I've never made a semi-raw sauce like that. Long slow cooking brings out a lot of flavor in the tomatoes ... as well as evaporating a lot of the water in them. You might do like Chryss and use drained jarred tomatoes.

Depending on how sweet the tomatoes are, most pasta sauce recipes (even old recipes that would've assumed you could get good ripe tomatoes) call for a teaspoon or so of sugar. You don't really taste it as sugar, but it brings out a lot of flavor.

A pinch of hot red pepper can bring out a lot of flavor too, even if you don't add enough that it tastes hot.