The user experience could be drastically increased by making the form validation
messages more concise and generally putting more thought into the (server-side)
validation.

Currently, if you submit the form the only message is something like: “The
address you specified for ‘Billing’ contains invalid information, or is
incomplete. Please ensure that all required fields are completed.”, and no
information whatsoever about which field is invalid (there are not even style
hooks (e. g. classes) on the elements to make them stand out visually) is
provided. Even for the developer that implements the module and templates it’s
not clear which fields are required in the first place (in the sample template
all fields have a class “required” assigned to them even though clearly not all
fields are required).

There are two great articles about this subject, and how great form validation
can work at

http://www.smashingmagazine.com/2012/06/27/form-field-validation-errors-only-approach/

and

http://webdesign.tutsplus.com/articles/user-experience-designing-form-validation-the-right-way--webdesign-681

Especially in e-commerce checkout forms a good user experience is crucial. Would
be nice if this could be implemented (on the sever side – for the client-side
validation the developer that implements the forms should be responsible
themselves)