What does the future hold?
As Artanis pointed out* that StarCraft is not WarCraft in space, and StarCraft II will remain faithful to its roots. Mission briefings are back, transformed into the new story mode reminiscent of the old, now almost extinct, adventure games. Players will be able to decide whether to jump into next mission right away or to witness character interactions, watch galactic news, and immerse themselves deep into the story and StarCraft lore.
Story mode is where you'll get to know the heroes such as Jim Raynor or Tychus Findlay closely, thus their presence in actual maps will be more scarce. You'll still be able to control them in missions where they can make a difference, but they will stay off-map in great battles, where the ground trembles under the stampede of Ultralisks, the sky is filled with Battlecruisers, and Yamato bolts are vaporizing the very air that is sparkling with Protoss psi-storms.
In such situation a single marine isn't going to be a lot of help, no matter how badass he is. Death of a hero will mean instant defeat as before, but new mechanics will come to play to make sure players don't leave them to rot in safety of their base. For example, Chris Metzen mentioned a gimmick, where instead of dying right away, your hero will drop to his knees and wait for a medic to revive him.
The regular units won't be alone on the battlefield, though. You will be able to hire a variety of mercenaries in between missions and then subsequently recruit them from Merc Haven. These unique units will boast improved stats and will therefore fulfil the role of heroes from StarCraft I, with one important difference: they are expendable. If they die, tough luck (for them). You'll have to finish the mission without them, but you'll be able to hire new mercs for the next mission.
The mercs don't gain any experience or upgrades, so there's no point to keep them sitting in your base. Let them go in a blaze of glory! That's what being a merc is all about, after all. Their role will probably be very similar to the Zerg "hero" units in StarCraft and Brood War, such as the Hunter Killers.
Blizzard has time and time again proven that they can learn from their past, return to interesting mechanics and use them in a new, innovative ways. From the recent interviews we know, that they've put quite some thought into the role of the hero units. One of the strongest parts of Blizzard's games has always been story and with StarCraft II we can look forward to really deep characters and relationships between them developed both on and off the battlefield. These characters may bot be present on every map, but when they do, they will lead the action and show that they are really worthy of being called heroes.