"We're middle-men (and women), glorified translators, and we're in the unbelievably fortunate position that programming computers is still much too hard for the average user of those computers, so we translate wishes and domain knowledge into a mental representation of machinery that performs some useful task."
The business logic is "the hard part" only in terms of client communication and project management, and then only barely.
Learning that "an Account Rep has Clients", and then modelling and implementing that, is ... not challenging? Three lines of ActiveRecord ('Duplo')?
They both vary tremendously depending on the problem, but on the whole, 'javascript widgets' trumps 'business logic' hands down. The fact that most business logic is simple is why you can glue things together so fast in Rails.
Olark's whole business is making a single javascript widget, and last I checked, they're still hiring.
Making things behave as expected in as heterogenous environment as the browser can be enormously complex as the sophistication and scope of the UI you present the user grows.