This FriendFeed thread regarding the Wikipedia licensing vote has stirred up an old hornet’s nest of issues surrounding copyleft and noncommercial clauses in Open licenses. As I said in the thread, I get most of my ideas on this topic from David Wiley, and have posted about those ideas before. Herewith another attempt to organize and clarify my thoughts, as much for my own benefit as anything:
1. The purpose of Open licensing is to enable the following (this is straight from David’s Open Education License draft, about which more later):

• Reuse – Use the work verbatim, just exactly as you found it
• Rework – Alter or transform the work so that it better meets your needs
• Remix – Combine the (verbatim or altered) work with other works to better meet your needs
• Redistribute – Share the verbatim work, the reworked work, or the remixed work with others

2. The purpose of restrictive clauses in such licensing is to prevent specific types of reuse, rework, remix and/or redistribution:

2a. Copyleft prevents future copyright lockup by requiring that all downstream (reworked or remixed) works be similarly licensed.
2b. Noncommercial clauses prevent profitmaking, and are complicated, and I’m not getting any further into it than that right now. (Maybe later, if my brain doesn’t melt.)

3. Although copyleft and NC clauses achieve their own immediate goals, widespread license incompatibility¹ means that they often (perhaps usually) defeat part of the larger purpose of Open licensing. The use case where this is most prominent is remix², since reuse and redistribution of individual copylefted or NC-licensed works or their derivatives is usually just a matter of retaining the original license. But multiple works can only be recombined into new works if their respective licenses are compatible — otherwise, there’s no licensing option for the remix that doesn’t violate the licensing terms of at least one of the ingredients. Not only that, but if any of the works in the mix carries a copyleft license, that license takes over the entire remix and everything downstream of it, thus propagating the incompatibility problem.
4. One last thing: could copyleft be saved from itself? What if someone wanted copyleft protection, without the compatibility issues? Creative Commons is already beginning to build the only solution I can think of: widespread interoperability agreements between existing and any newly developed copyleft licenses. CC-BY-SA 3.0 contains the following clause:

You may distribute, publicly display, publicly perform, or publicly digitally perform a Derivative Work only under: (i) the terms of this License; (ii) a later version of this License with the same License Elements as this License; (iii) either the Creative Commons (Unported) license or a Creative Commons jurisdiction license (either this or a later license version) that contains the same License Elements as this License (e.g. Attribution-ShareAlike 3.0 (Unported)); (iv) a Creative Commons Compatible License.

where (iv) is defined as

a license that is listed at http://creativecommons.org/compatiblelicenses

Sadly, the cupboard remains bare so far:

Please note that to date, Creative Commons has not approved any licenses for compatibility; however, we are hopeful that we may be able to do so in the future. If you would like to discuss the possible compatibility of your license with a Creative Commons license, please email us at info@creativecommons.org.

I am personally persuaded that the Public Domain is the best way out of the copyleft trap, which is why I use CCZero for everything I make.

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¹ Among CC licenses, there is only about 33% compatibility, and that drops to 20% among NC and SA versions — including self-compatibility^*:

Restrictive (NC, SA) versions currently account for around 80% of worldwide CC licence uptake. Once you start factoring in the dozens and dozens of other Open/Free licenses out there, it only gets worse. The FSF and OSI maintain lists of licenses and compatibilities (here and here, respectively), and wikipedia includes a couple of fairly extensive comparison tables. Speaking of Wikipedia, the world’s favourite online encyclopaedia is currently released under the GNU Free Documentation License, which is not compatible with any CC license except Public Domain though it does allow transition to CC-BY-SA. If the current vote on that transition is “yes”, that will be a step forward — but it will still leave Wikipedia with the compatibility problems shown in the figure above. Exploration of compatibility issues with all the other Free/Open licenses is left as an exercise, etc.
^* from here and here; green indicates compatibility, light green indicates possible compatibility — some disagreement between sources.

²This is why I consider David’s “Four R’s” formulation so important, because it makes a clear distinction between rework and remix that is essential to understanding the aims and implementation of Open licenses.