Typically, the objective required to finish the mission gave me a nice chunk of Levels, and optional objectives were available for smaller, bite-sized bonuses. Kerrigan is your persistent hero unit throughout the campaign, and she gains 'Levels' (as opposed to experience) for completing mission objectives. Speaking of levels, Heart of the Swarm is doing things a little differently than, say, Warcraft III. This, oddly, is not modeled as an organic part of the ship, but rather a more traditional menu display. There's also the Kerrigan upgrade screen, which will let you select talents once you've earned enough levels in the campaign missions. The evolution pit is where you will evolve your units (more on this later) to create a custom Zerg army.
Aside from the bridge, which is some kind of toothed orifice (I hope it's a mouth please let it just be a mouth.) where you will interact with your advisers (and, occasionally, prisoners), I was able to skitter over to a couple other areas.
