Quality. Something difficult to find in a knife style that's dominated by the so-called China cheapies. Benchmade is pretty much the only stand-out, but its product lines are long discontinued.
If you're just starting out with balis, check out Popular Link (www.popularlink.com) and pick up cut-price (but not cut-quality, at least no more than usual) Jaguars. You can get a whole stack to practice with and abuse until the new Benchmade lines come out.
The main problem with traditional-style balisongs is that they lack some of the engineering advantages (such as the kick, which keeps the cutting edge from smashing against the liner) of redesigned knives. Some of the beautiful Filipino hand-crafts are pretty sorry as functional knives after a few flips.
A big problem with the cheap practice Jaguars that I favor is that a breaking pin will generally have catastrophic results (I've been very lucky), but is difficult to detect. I've yet to have problems with my latest order (the newer Jaguars supposedly have better pins), but I also haven't put any real stress or time on them.
