Youtube – Gray exclamation mark keeps appearing on YouTube

It is certainly buffering issue, guess your Internet speed is slow, would recommend you that whenever you try to play any video, just pause it, let it buffer, when you see a grey line filled up, you can start playing that video.

Firefox add-on SmartVideo for YouTube will do just that. You can even tell it to either start the video when you want. You can just open the link in a new tab, and just come back to it when you're ready to watch the vid. It has other useful options as well.

Edit: SmartVideo can be a bit buggy. As to 720p videos being buffered, I think it's because all videos in 720p (or higher? not sure) are already on WebM format. It's my default setting. It's the fastest video setting to load, faster than a 480p flv file.

Edit2: SmartVideo is also available for Google Chrome: https://chrome.google.com/webstore/detail/smartvideo-for-youtube/lnkdbjbjpnpjeciipoaflmpcddinpjjp?hl=en