The apps which are there tend to be poor. Facebook, Twitter, Mint, various banking apps, and others are either lacking features now or way behind in getting new features, on top of being shoddy implementations as-is.
The games story is basically pathetic.
The most important thing though is whenever something new comes out. It happens countless times per year - some unexpected fast rising app hits cultural zeitgeist and takes the world by storm. Whether it's Instagram or Draw Something or Angry Birds Space or something else. Whether it sticks around for a week or a month or years. Windows Phone never has it, or gets it late after no one cares anymore.
It doesn't matter for everyone, but it's undoubtedly important to millions for those apps to be there at launch or close. Microsoft's making a big deal out of getting some Zynga games and Angry Birds at the launch of Windows Phone 8 (or whatever it is), but by that time few will care anymore and the world will have moved onto something else.
It's very important to prospective buyers, and the situation absolutely sucks right now. What I'm hoping is that the enablement of native development and popular engines like Unreal (probably) and Unity (confirmed) will make it much more likely for more apps to come to Windows Phone, and faster, and the Silverlight platform won't make them suck anymore.