Mountain Bike Frame Design Rules of Thumb:

The fundamental problem in bike design is balancing the requirements of ascent and descent: In this, shifting centre of gravity when braking downhill or pedaling uphill is the main concern. This is adjusted with the overall length and height of the frame, as well as seat position, stem extension and fork rake.  In descent, the centre of gravity is best located behind and below where it should be for ascent. Many riders naturally slide to the back of their saddle, or actually behind it on extreme downhill grades. If the centre of gravity is too far back, however, a bike may descend well but be jumpy or even tend to flip over when being pedaled uphill. The trick is to find a balance that accommodates both ascent and descent, and the fact here is that everyone's requirements are subtly different. 

The rider's centre of gravity shifts continuously while riding: The shifts are greatest during acceleration and braking. A longer wheelbase accommodates larger shifts and is a more stable ride. The currently widespread practice of fitting short frames to taller riders by using long stems creates unstable bikes. This is particularly true when braking hard in descent, where a long stem can cause the riders centre of gravity to shift over the front wheel, destabilizing the bike. A proportionally longer frame and shorter stem provides more predictable handling and is far more stable. There is a limit here, and BMX style shorties on very long frames don't really work, at least for me. My 18 inch titanium frame is made in both a short and long version to properly fit most average sized riders.

Ride the slopes, don't slope the ride:  Sloping top tubes and extended seat posts are a part of the visual language of mountain biking. A gently sloped tube does provide the flexibility to lower centre of gravity in descent and a lower stand over height for a given frame length. It may also allow a slightly lower saddle position for extreme riding conditions. These are good things, but overly sloped frames produce ridiculous seat post extensions for proper road position, and the long lever arm of the post is both springy and loads the seat stay unnecessarily. Both very short and highly sloped frames are really nothing more than exercises in simplified mass production, making it possible for one frame size to fit a wider number of consumers. They are not valid designs, except perhaps for children and very small adults.

Keep the tubes straight and round: The round tube is the strongest and stiffest structural form. Ovalizing, bending, using square sections or otherwise unnecessarily messing around with this form will produce a frame that is heavier, more flexible or weaker than a frame made from straight round tubes. My favourite example is the curvy “S” bend chain and seat stays you see in many designs. Bends introduce flex points, which are particularly undesirable in the rear triangle, and unnecessarily compromise power transfer by increasing the springiness of the frame.

Simpler is better: Always. Bicycles are simple machines. If an "innovation" looks like it answers a problem you didn't know you had, its probably a gimmick.