This weekend my coven will be celebrating our first "outdoor" sabbat. I know that a lot of groups exclusively meet outside but that's never really been an option for us. While my wife and I are lucky enough to live in a house, there's another person living in our backyard. He's not a living in a tent or anything like that, but he does occupy a studio-like living space attached to the garage. I doubt he wants to listen to us chant in the backyard while he's trying to sleep. 
While I do share a backyard the garden spots are all mine and with the corn already over six feet it feels pretty magical. It may not be with the coven, but every time I water my garden (with grey water from the shower) I feel like I'm at least performing a private ritual. I talk to my sunflowers, implore my pumpkins to grow, and stop to bow at Aphrodite-Chicago of the Lemon Tree. My garden is ia magical place, but it's a magical place for mostly "just me" (and sometimes my wife when she checks on things).
Private observances are in some ways the meat and potatoes of Modern Paganism. If we add up every little prayer and devotion we make over the course of a lifetime those moments will probably far outweigh ritual time with the coven. Some folks are entirely happy with a lifetime of those private moments, but I need coven ritual. It would be great to circle in the shadow of my artichoke plant, but it just doesn't seem like a good idea, so indoors it is.
Meeting indoors has never been a deal breaker for anyone in our coven. We have a lovely ritual room and it crackles with energy even when we aren't doing ritual there. For many of the sabbats meeting indoors makes sense too, from Samhain-Ostara I generally want to be inside. I have Druid friends who ritualize outdoors even at Yule (and they live in Michigan), to that I say no thank you, there's a reason I live in a house.
Members of our coven certainly do ritual in the outdoors. All of us are involved with local, larger (and public) groups that meet in the outside for much of the year. But in some ways those larger, eclectic rituals feel more like family reunions or community potlucks than serious ritual (especially during the Summer).
While I love my garden and I love my ritual room, Midsummer has always felt like a ritual that should be celebrated outdoors. Last year I accomplished this by conducting a small (rather non-magical) part of our solstice ritual around our portable fire-pit; into the fire our grape-vine solar wheels went and all without freaking out any of the neighbors. While we were able to see Mama Moon for a few minutes, it was still an indoor rite, and a warm one. Once the circle was cast the mercury hit about 90 degrees. A little sweat is good, a bunch of it falling off my brow and into the ritual wine is bad.
The newest members of our coven live about an hour from the covenstead (and you know our rituals are good if people are traveling that far for them) in a big house surrounded by redwood trees. When they offered to let us do some rituals there this summer I jumped at the chance. Midsummer and Lammas in the woods!?! Yes please. Oh and stars . . . . . no light pollution to worry about.
The opportunity to take the coven outside got me thinking about the differences between indoor and outdoor rites. There are things I like about both, and while indoor rites can feel very clinical to some, I think they sometimes have an ambience worth appreciating.
There are things I like about working indoors, the biggest one of them is that it just feels very "occult" to me. I like the "secret society" aspects of the Craft and it's hard to imagine clandestine rites taking place outdoors (especially in a place as packed full of people as Silicon Valley). Indoor rites also bring to mind groups like the Golden Dawn and the Freemasons who all met (and still meet) in very specific spaces. For all of the earthiness found in Modern Wicca, Gerald Gardner was still initiated in a house (most likely Mill House, one of the three homes of Dorothy Clutterbuck).
I also like the control an indoor ritual provides. I know just how "clean" the space is, and with a dedicated ritual room there's always at least a little energy floating around inside of it. I also don't have to worry about candles getting blown out or not being able to read my lines (sorry, I'm a reader, not a memorizer; we even keep cheat sheets of our quarter calls taped to the walls of the ritual room). It's also where most of my magical stuff is. Did I forget my athame? Oh wait, it's right here. 
(Backyard or patio ritual work occupies a middle space between between the worlds of indoor and out. It generally provides the control of an indoor working space with the fresh air and moonlight of an outdoor one. On the downside it might very well sound like a city: trains, cars, horns, neighbors yelling, barking dogs, etc. In Michigan with our half an acre of yard this was easily done, alas, those days ended when we headed towards the West Coast.)
There's something very primal about ritual outside and it most certainly brings out my inner-Pagan. I always feel my most Pagan when outdoors. On those few occasions when I've been able to do ritual far away from most of civilization I've always marveled at how "barrier-free" it felt. There's no wood or carpet between my feet and the earth, or strange wires under the ground carrying in cable television signals. The moon, the stars, the planets, none of them have to fight through a haze of light pollution in order to be seen.
I sometimes can't see my circle-mates depending on the lighting, but I can usually feel other things. Not just the gods (we feel those just about everywhere), but the fey and all the other hidden folk who aren't fans of city life and strip-malls. If the Earth has a pulse, I feel it best outside.
Ritual work outside is generally delightful, but it's not perfect. A broiling 100 degree sun is no place for a coven, both sunburns and cancer are things I actively try to avoid. And I've been involved in some rites that had more mosquitos than fey, or even coveners. There are particularly muggy nights when citronella and deet aren't enough, and those evenings I'd rather be indoors. I love nature, but humans have lived in caves and other dwellings for tens of thousands of years now, sometimes they just come in handy.
The majority if my rituals take place indoors, and I don't think it makes me a bad Pagan or Witch. Ideally a working space should be one where people feel comfortable and won't be interrupted. My ritual room gives me those things, but it's nice to know that at least in the Summer I have an outdoor option to feed my inner Goat-Boy.