I found myself asking the same question, and then found http://basecamp.com/breeze/compare. It seems that they're offering easier set-up in return for the $10.
The pictures helpfully clarify that Google Groups has a couple of extra dropdowns and a captcha to complete before sending your email whereas the 37signals minimalist credit card form has um... a couple of dropdowns and a long number to copy accurately.
Not to mention the processes are exaggerated, and outright deceptive.
Funny, why do we need to spend multiple steps "signing in" to Yahoo Groups, and not Breeze? Because, sorry, 37signals, the opening page for Basecamp Breeze is not "Create A List".
But I guess you need to exaggerate for effect (this is the same company that defended Apple against the "sequences shortened" lawsuits).
Oh, and "(1)", "(2)", "Create" on the Breeze create a list page...
"(1), (2), (3), (4), 'Create'"... that's more accurate.
So... "look how many steps our competitors take, for their free offering. Ours is less steps (because we'll bundle several of them onto each page), so pay us $10 for that convenience, DHH needs a new supercar!" - sorry, I couldn't resist the snark, but this "marketing" is dishonest. And the blog entry as much so. In fact, I could sum it up largely as:
1) throw dart
2) if target hit, great, if target missed, retry step 1