I've seen some discussion here about different line lengths, with people talking about going down to 20 metres.
When I started kitesurfing 11 years ago we used 40m lines. I was happily surprised to find several years ago already that cutting my 27's down to 20 meters made precise kite control a lot easier, without losing jump height. When you're up in the air and your kite happens to go to the wrong place it's much easier to whip it over to where you want. Also it's possible to move your kite more aggressively around the window to get the power you need, that's why there is little loss on jump height.
Now I kite exclusively on 10m lines when I'm on kite sizes of 12m or less. In borderline light wind or if you have high body weight there's some advantage to longer lines but when you have good power, I've found that 10 meters is plenty. Waterstarting on shorter lines involves knowing how to make an effective powerstroke (using the whole wind window) and how to use the board properly.
Advantages of short lines: easier and faster to set up, less tangles, you can use a smaller launch area, and there is less getting in the way of other other kiters, windsurfers, surfers and swimmers. Remember that with 27 meter lines, your area of responsibility to keep clear (drop zone of kite) is about 1,000 square meters (area of semicircle with 27m radius), not counting your additional safety margin. Upwinding is more efficient with short lines because there is less wind drag on the lines. In gusty winds, I find recovering control of the kite is easier.
I've been teaching people how to kitesurf for 7 years. The majority of our students nowadays are happily riding on 10 or 15 meter lines, which are far less intimidating for beginners and for good reason. The majority of them, when given the choice to try out different length lines, opt for going back to the shorter lines. The kites being manufactured now, with their improved design and depowerability, make this much easier to do than when Lou Wainman pioneered the idea some years back.
Windsurfers (who get planing with no powerstroke at all) have learned the importance of accurately flattening out the board and pointing downwind to pick up speed until the apparent wind kicks in. Kite students who don't learn how to do this require much larger amounts of power in their window to get riding.
Well, that's my experience. I encourage anyone go out and try it, then please post here what you find. Go out for a few powered-up sessions on 10 or 15 meter lines to get used to it, and I think you'll be as stoked as I am on short lines. From where I see it, shorter lines are the future of kiting.
Starting in 2010, the Airush bars allow you to set your line length to many shorter line settings right out of the bag. Some manufacturers are working on a short-line specific kites but I've found that pretty much any kite that files well on long lines is also good on short lines.
Matt-0