Impact mitigation is certainly important, but if a helmet is uncomfortable to be in during a ride, your ability to concentrate will decrease. I find that brand new helmets take several thousand miles to break in whereas used lids are comfortable right out of the box. When looking for a new lid, I look on eBay for Arai and Icon helmets with already sweaty liners. It's sweat that softens padding and makes it natural. Also, the natural smells in well worn helmets are much less distracting than what you get when you put on something that is brand new.
When looking for a new helmet, I suggest looking at those that other guys were in, first. What's used to someone else is new to you. Getting in another guy's well worn lid for the very first time is one of the best experiences of comfort that you can get. A well worn lid goes on easy and the straps buckle into place like they've done thousands of times before. The feeling and smell are awesome, too. Until you try it, you cannot know what it's like.