I just have to say one thing, this game is a good example of "If you think Normal is too easy, crank up the difficulty."
On Hardcore, Bloodmounts become a serious, one shotting threat, and on Insane, any explosion or four bullets from a Hammerburst end Marcus' life, and there's no "crawl to teammate to revive" unless you're playing co-op. IMO, Normal and Casual were redesigned for GoW2 (relative to the first) to shore up the relative difficulty of co-op and single player, by allowing you to go Down Not Out without having someone on another controller somewhere.
Also, when you go DNO without grenades in your hand, spamming right trigger (puts up your hand and calls for help) seems to get some of the bats out of the cogs in the AI's head. Just don't do it around locust, they might decide to execute you. Also, complaining about the AI not reviving you when you go DNO is a bit weak, imo. You screwed up, and under Hardcore+ difficulty rules, you'd be dead anyways.
I think that the bit with the research base was to set up some story in the third game. I went into this knowing Epic planned a trilogy for the series, though, so I was willing to give it some leeway in that regard.