Most of us here take user experience very seriously, and we answer questions with testing, convention, design pattern, white spacing, grouping, similarity, simplicity, proximity, consistency and much more as core business. But a quality for a user may also be to have fun accomplishing tasks. Now, is "having fun" a quality of User Experience?
How fun, rewarding, or satisfying an experience is can have a huge impact on the engagement and ultimate success of a site. Engaged users are far more likely to overlook friction and keep participating. Fun increases user motivation.
Stephen Anderson, paraphrasing Joshua Porter, sums up the relationship between usability and motivation nicely:
A few references:
Fun can be a great part of a good User Experience and is a form of Emotional Design. Fun can make an experience great because of the memories it causes and how pleasant it can make mundane tasks or even error messages.
However, fun is a possible form of emotional design, and it doesn't speak to all users. Fun is more applicable in consumer applications or landing pages than it is in business products or during complex tasks.
Fun isn't always good; personalities clash in applications just like people, and there are some situations where fun would simply be an annoyance or unwanted. This is most typically the case when software is supposed to be professional and efficient. When entering payroll for a large company, most employees don't want to have fun.
If "fun" gets in the way of using your product it's bad. It might turn users away even if it isn't obtrusive at all; you have to be very careful when setting your tone to fit your audience.
Think of the experience video game designers have in mind for their users. Likely, game designers want their users to "have fun", take an amazing journey, or another phrase with fluffy adjectives. As a designer, you guide the User Experience. You decided whether you want it be a quick and seamless experience, or if you want your design to be something other than "very usable."
I remember a talk I heard by a designer at Frog (unfortunately I can't find a link) about some vacation planning website. The clients wanted users to have an emotional connection with the destination just through using the site. For the designers, this meant evoking "emotion" was top priority, even above usability. The same could be said for an application being "fun".