Collocation of development team members from various disciplines (design, development, test) has many benefits. If you've been working in an agile environment, you know about the enhanced communication. But don't stop there! Get your teammates to review your work before you check it in, and you get crucial feedback and catch some bugs ASAP.
Just because a feature looks cool in a 5-minute demo doesn't mean it will do the job in real life. It might not scale for large amounts of data, for example. So make sure you're designing, developing, and testing for the real world! I saw this principle in action recently when I was called on to debug a performance problem....