Seth Godin has a post about the impact of enthusiasm. He claims that enthusiasm makes a great impact and improves an experience in business. While I agree with him that it is far more pleasant to be dealing with people who are passionate about what they do, I believe it is fundamental to choose the context and the audience in which it is OK to be enthusiastic.
Who has not seen an enthusiastic business person becoming a nuisance simply because the audience is not ready to follow or the context is not appropriate? In fact, more than a choice of context, enthusiasm is something you share with your business partners, so if you want to play by this tune the key is to be able to adjust yourself to the mood and style of your counterpart(s) and to help them make the transition from the state in which they are to a state in which they can actually take your enthusiasm...
Of course, a prerequisite for being credible as an enthusiastic business person is to actually be passionate about what you do, to be committed to your projects... Which brings us to the importance of actually doing something one likes rather than something that has the only virtue of being acceptable to a social group (especially if the social group is overly conservative - if Picasso had listened to conservative types quite a few works of art would not be around for us to enjoy them).
Doing something with passion in business is another way of saying that you are actually pursuing a business quest.