Living in Northern VA I try to make sure I go to Wolf Trap at least once a season for several reasons:
1) Small Venue, great sound! The giant wooden structure is built into the ground and the sound is great, whether you're in seats or in the lawn.
2) BYO Everything!! Its so nice that they allow you to bring a cooler in with food/beer/drinks/etc so you can actually have a picnic in the lawn while enjoying a show. You can't bring the cooler into the pavilion but several times I've just left my blanket/cooler in the lawn and it was there when I returned after the concert ended.
3) Friendly staff - most of the ushers and people working at Wolf Trap are friendly and knowledgeable about the history of the place.
The only downside is that it is a smaller venue so you're not going to get a really popular band to play there. If you can find some unknowns (like John Butler Trio - AWESOME band!!) or stars past their prime (like Steve Miller, Lionel Richie, etc) its well worth it.