A pine futon frame should be strong and designed to last many years, most of our members would look for quality aspects in their frames such as the dimensions of the timber used. Slats should be around 7.5cm+ wide and 1.5cm+ thick and made from a good quality pine such as redwood pines. The underails (this is the part that supports the frame) should be of a similar dimension or look for thicker underails if the base is for everyday use as a bed.
The other factor to consider is how the futon base sits when open as a bed. Many 'A' frames are designed with small front 'feet' in order to provide a comfortable sitting angle when the frame is in a seat position, however when opened as a bed this means that one end of the frame will be slightly raised, not really a problem for visiting guests, but worth considering if it's your main bed. The serious futon user should check out bases featuring integral feet on all the underails (such as Futon World's Acer or Osumi). This ensures that when the base is open in a bed position it forms a slightly raised, level, sleeping area, perfect for everyday bed use.
If you need any further advice on selecting a frame post you questions or comments here, and we will offer advice.