"Honest question: how much innovation is there in this field? Is there a steady stream of new magic tricks?"There's a huge amount of innovation going on in magic, and there are new tricks coming out all the time. Especially with the advances in technology and science these days, there are a lot of fresh effects being developed constantly.
Of course, many of those tend to be new approaches to old problems: a new method to levitate something, a new way to make something disappear or reappear, etc..
Some magicians prefer to stick with the tried and true, century-old (or even millennia-old) tools and techniques. But others (especially the younger generation) really embrace new developments and new technology.
There's a real thirst for making one's act fresh and original, and for amazing the audience with things they've never seen before. And magic suppliers and magicians are constantly developing new products to fill those needs.
"The extralegal protection system they've developed - essentially a guild system - doesn't seem like a terribly promising candidate for broad application. Among other things, it's more effective than the patent system at stopping ideas getting into the public domain, ever. Whatever its other drawbacks, a patent means an idea will eventually make its way into the public domain."
If a patent is of something truly original and of something that really would not have been easily developed by someone else, then yes, patents could potentially have value to the public.
But unfortunately, patents are constantly granted for things which are really obvious and for which prior art existed, or which would have easily been developed anyway as soon as anyone with half a clue set their mind to it.
Also, at least in the case of the fast-paced computer field, by the time patents expire the technology that was patented is often completely obsolete and useless. Who really cares if the patents on punch card technology has expired by now? We've moved far beyond that to the extent that it's useless.
This isn't quite true in the magic world, where ancient tricks still retain the ability to shock and amaze people who've never seen them before. Of course, there are fads in magic, and certain tricks become so popular that they're useless... for a period of time, until someone rediscovers them again. And, despite the best efforts of magicians, there are many many copycats. A hot new trick will often be duplicated and reverse engineered in no time.