If you're worried about the short development time, look at it this way.
It took them years to craft the universe, the story, the characters, and gameplay of the first game. Several years of backstory and design, finally accomplished in November 2007. Boom, ME1.
After that, they looked at what worked and what didn't and made tweaks to the story-telling, gameplay, graphics, etc. No need to spend years crafting the universe, it's already established. Most of the work goes into improving the engine, looking at feedback, and developing the gameplay. ME2 released in February 2010. Time between sequels: 2 years, 3 months.
Now all that's left is to wrap up the story. Make the necessary upgrades to the engine. At this point they have a good idea of what works and what doesn't. Take the best parts of both games (story-telling, gameplay, etc.) plus 4 years of feedback, and hone your product into the most polished game possible. ME3 will be released in March 2012.
Time between sequels: 2 years, 1 month.
So yeah, did you like ME2? Considering that the turnaround time between 1-2 and 2-3 is almost identical, are you really that worried?