Per this tweet, it seems that 1Password is designed to obey the web form fields if they are present on the page. If not, 1Password reasonably will try to find appropriate fields (registration versus login forms are different; it makes sense to be as compatible as possible). However, even when all fields are properly present on the page, 1Password will additionally attempt to fill in every text field anyway.
This poses a major problem in WordPress, which pops up a login in an iframe when your cookies expire. Once the iframe closes on auto-submit, all text fields will suddenly be filled with your username. This would affect editing a post, editing settings, etc. Very messy. I would expect this would affect any site with a pop-up login that doesn't require a full-page refresh, and in general any site implementing things without full page refreshes.
There are many possible fixes here. Probably the most limited fix (one with the least chance of breaking things for other users) is, if all web form fields match within the iframe, only touch other fields that are within the iframe. Obviously, this is limited and won't fix other side effects. It will, however, solve most difficulties I've seen reported with WordPress. (There are other issues, of course. If a login field has an additional two-factor authentication text input, 1Password will fill it, even though it wasn't configured to do so.)
I'm a WordPress lead developer; here's the ticket we are using to track this issue. There are steps to reproduce there that are specific to WordPress.
Tested with 1Password for Mac 4.1.2 (on Mavericks), Chrome extension 22.214.171.124, Firefox extension 4.0.1.
The tweet I linked to above was in reply to a request for some technical details on how exactly 1Password determines which form fields to fill. We may need to make separate adjustments for saving the right credentials on user creation and site installation (it grabs the email instead of the login). These kinds of details are very important to web application developers.
Here's a simple HTML form example. Note the other input fields aren't even inside a form tag. 1Password doesn't care, even if the web form details are properly configured for "log" and "pwd". They can be in the same form, or in a separate form, or in another frame, it doesn't matter, 1Password will fill them. That includes filling over other data that is already present, which can potentially be destructive. :-(