I was looking at some of the wifi 2.4Ghz yagi antennas available on ebay. Most of them claim some crazy dbi gain numbers. They also come with a fairly short cable with an sma connector, 6ft most.
1. Would having a 15 ft cable defeat the purpose in gain taking into the consideration the loss of signal with such a high frequency? and such a small diameter coax?
Yep. 15 feet of RG-196/A, which is a particular (0.080 inch) small diameter teflon coax has about 11dB of loss at 2.4GHz.
2. I understand that more elements the better for gain, most of the longer/bigger ones have a fairly similar amount of radiators? why?
Yagi gain is actually more of a function of boom length than it is of element number. Now, you can't have a ten wavelength boom with three elements... so you have to add elements as you add length. But if you have two yagis with the same boom length, it's not necessarily the one with more elements that's better.
3. What to look for in general? Do you have any experience with these?
No idea, but I would look for people who are trying to establish long range links ... "WiFi DX" type enthusiasts and see what they recommend based on actual testing under really marginal conditions.
4. There is some with Wifi controllers built into the Yagi, all is needed is a USB cable running into the computer, do you think that would be a better choice looking at signal amplification by the Wifi controller? I understand USB has a limitation, so i would be looking at roughly 10 ft of usb running towards my computer.
This is a better idea than coax, provided that the WiFi thing that comes with the antenna does what you need it to do. Mounting the radio (or part of it, like a downconverter to a low frequency IF) at the antenna to minimize feedline length is a common practice in the microwave world.