First the linkage to Gmail and before I go into a detailed conversation about it, let me just preface it by saying I am a Google fanboy. There are few things Google does that doesn't work. However, one part consistent about Google is the relentlessness it is in interoperability with all of its products (except really Google Wave). Gmail, Reader, Docs, YouTube, AdSense, Voice, etc all can work together under one account and that's the first part of Buzz that hits Twitter right between the T's - Buzz has something that Twitter will never have - interoperability with dozens of other products you use every day.
Second is Google's mobile version of Buzz which makes it even more powerful. You see once you sign in it asks to use your location. Now you can find people nearby with Buzz activated. Also, if you visit a Google Places you can see any buzz about that location and also leave your own Buzz. Finally, you can also leave a Voice Buzz from your Android or iPhone.
Twitter has absolutely no answer for Buzz. Sadly, over time Twitter will either be bought by someone desperate (Yahoo comes to mind) or will fade into the sunset like MySpace or Geo Cities. I don't see Facebook going downhill because of Buzz, but who knows. Right now it just looks like a Twitter killer to me. Oh before you jump right in, make sure you read this great piece by Molly Wood about Privacy issues with Buzz...