I always wanted the company to focus on a mission about being "useful" - like the digital WD-40/duck tape of your life. Finance, news, fantasy-sports, email, messenger, groups, My Yahoo!, Delicious, Flickr... They're all useful. They could have continued to happily and profitably offer all of these services - and expanded to dozens more - under the mantra of being useful to our every day lives, but Yahoo! was never content with that. They wanted to be eBay, or Google, or Facebook, or YouTube or whoever else was a big player at the time, instead of focusing on the customer loyalty they had already generated.
Having that clear core vision/mission seems like a small thing, but not having one crippled Yahoo! and caused infighting, turnover and confusion that eventually killed it. (Oh, and also Mayer was an idiot.)