The most important theme I observed at GDC 2011 was: good tools are what make the difference. Funny considering I didn't attend any talks that were specifically about tools.
Playdead's level design talk revealed that their real-time level-building tools were what allowed them to quickly create, test, and iterate on Limbo's puzzles. This speed of development allowed them to create enough content that they were able to discard approximately 70% of their puzzles and only keep the cream of the crop.
In the Halo Reach networking talk, David Aldridge said that debugging tools are what allowed Bungie to reduce bandwidth requirements to less than 20% of Halo 3's. Adding the ability to splice network profiling data into their recorded game sessions let them play back traffic data along with game data. This profiling data could be viewed at any point in the replay using a graphical overlay. Alridge said these tools took about 6 weeks to implement.
In her SPU-Based Deferred Shading talk, Christina Coffin stated that good debug visualizations are key to both guiding their content optimization and validating the aggressive culling required by their complex scenes.
While development is in full swing, it's often hard to convince yourself and your team to invest the time and resources necessary to build an effective tool suite. These talks served as a great reminder that investing in tools can enable you to produce features and content that would not otherwise be possible.