As far as describing the beer you're doing just fine. Roasted, watery, thin, earthy... all great descriptors.
I've had a few beers that were thin and really lacking in the mid palate, come to think of it they were extract kits as well. My solution was to make bigger beers.
Something I've been doing lately is adding 1/4 cup maltodextrin with 15 minutes left in the boil. This really seems to help with body and head retention.
For flavor, steeping some Crystal/Caramel specialty grains before the boil can really help fill in the gaps in flavor. Just pick a roast level that's appropriate for the beer you're making. 10L = Light Gold, 60L = dark amber. 2lb 60L would work just fine in a porter.
Good luck to ya - hope this helps some.
"Drinking tripel makes me see double and act single."