I agree that it is a lame aspect of the new Safari. Partially lame for the presentation. And partially lame for the effect if one clicks on "Visit Page".
I have mostly (or always) have clicked Cancel, waited a few seconds, and then the page I'm trying to load eventually loads. I typically have 60+ tabs open, and I'm not about to force reload all of then. For those people who are just clicking on "Visit Page" I recommend clicking on Cancel instead and seeing what happens.
The error message always says that "webpages" are not responding, but I've had this problem when loading just a signal page. For it to use the plural of webpages is confusing/mis-leading.
Also, why can't it force reload just that specific tab?
This is the type of error message I'd expect to see in a Windows OS.
To understand why this comes up you need to understand how the Safari processes have been redesigned for version 5.1
Historically plugins have been a leading cause of many a Safari crash. So in Safari 4 Apple seperated Safari into 2 processes - one of Safari itself and it's renderer and the other for it's plugins to run in. This meant that if the plugin crashed it Safari would in theory continue to function.
This of course ignored the other ways in which a web page can crash a browser so in Safari 5.1 Apple separated things even further. Now Safari is separated into 3 processes - The Safari app comprimising of the windows , menus , tabs , bookmarks and URLs of what's loaded in each tab/window , our old friend from above WebkitPluginAgent and a new process just for the web page renderer. This new process is called "Safari Web Content"
What your seeing here is when the Safari Web Content process crashes on the content of a page for any reason.
Safari can detect when this happens and automatically restarts the process. When you click back into a window or tab where you had web page stuff loaded before it gets reloaded.