"Following in the footsteps of our new red and white Google+ icon, the +1 button is sporting a fresh coat of paint," informs Google.
While the new buttons are more consistent and include the Google+ branding, the old buttons are more colorful and more descriptive. "+1" is bigger and more obvious in the old buttons and that made them more clickable.
Here's the tiny +1 button from Google+:
... and here's the +1 button from Google Groups:
The small +1 buttons are not legible and it's not obvious that you're supposed to click them. As Fernando Fonseca says, "I understand the need to have a button that looks like the new logo but the problem is that a white background with a thin red line is hardly eye catching and hardly says 'Click me'."
You can compare the different versions of the +1 (v1) and +1 (v2) using the corresponding sprites. I'd choose the old buttons.
Update: There's still time to improve the buttons. Share your feedback here and here.
{ Thanks, Yu-Hsuan Lin. }