This version is mostly a bug and security fix release, with some new features and noticeable improvements :

The minimum required PHP version for Thelia 2.3.4 is PHP 5.5. Do not upgrade if your server is currently using a lower PHP version.
Admin Home page statistics improvements
Coupon list is now paginated, to manage more easily a large amount of coupons
Modules can now be protected and/or hidden, to prevent disastrous module deactivation. For example, the Smarty template engine integration module is protected and cannot be deactivated. It is also hidden. The Front office integration module is not protected, but hidden by default.
Uncaught exceptions are now logged in the Thelia log file.
An Image position field is now available in Carousel module configuration
The products template feature has been improved in several ways (see pull request #2363 for details)
“Coupon” substitutions are now available in the templates (documentation is here)
It is now possible to send a test email message from the Preview tab of mailing template modification page
The stock of a virtual product is now checked
Favicon, store logo and e-mail banner are now managed from the back-office -> Configuration -> Store
Requesting the view of a not visible product, category, contents, folder or brand now causes an HTTP 404 error instead of trying to display an empty view
And more !