First, let me point out the obvious: the GSL doesn't affect what you decide to write for your home campaign at all. AT ALL. What it does affect is the ability for third-party publishers to write 4e-compatible materials. As restrictive as the GSL is, there are going to be fewer people publishing for 4e. Which means you, as a player/DM, will have fewer choices when it comes to ready-to-use material. But if you're the sort of player/DM that never bought non-Wizards stuff anyway, it doesn't affect you at all, so you might as well ignore the rest of this blog post.
But if you ARE the sort of person who likes to see what other creative minds are doing with 4e, read on.
In short: wow, this is really, really restrictive. It looks like they really don't want people making third-party 4e products (and from what I've heard, this is true ... the people in charge think the OGL was a big mistake). Lemme take look at it section-by-section and point out any weirdness that comes to mind.
1. You have to write in for permission first, and if your contact info changes you have to update them else the license is forfeit. And they get to decide whether or not to approve you, so if you PO someone at Wizards for whatever reason ("he stole my wife!") you're outta luck.
2. Wizards can update the license any time they want, one-sidedly, with no warning and no announcement. If you don't like the new terms of the license (and who knows, they could be something really crazy like demanding a 50% royalty on each product), your only option is to stop selling licensed books ... except the license says that if you continue to publish after an update, it means you automatically accept the terms of that update.
4.1. You can't redefine any term in 4e. This used to be in the d20 STL but you could ignore it if you didn't use the d20STL ... now there's no avoiding it. So you can't make 4e-ish products like you could make 3e-ish products. Under this license there would be no Mutants & Masterminds, or True20, or Blue Rose, and so on.
5.5a. A licensed product cannot include a website ... does that mean I can't sell a ZIP file containing a PDF and a readme.html file with contact info for my company?
What is an "interactive product"? Does a PDF with embedded videos or sounds count?
What is a "miniature"? Do printable tokens count? What about fold-up "paper minis"?
What is a "character creator"? Does a random history generator with no game stats ("lower-class human brewer from the northlands, blond hair, brown eyes, tall, fat, scar under his left eye from a barfight") count?
5.5b. Similar to my last question, can I give advice (a process) of how to create the backstory of a character?
5.5d. If I can't refer to artwork in a core rulebook, does this mean I can't say "this character looks like the evil brother of the guy at the start PH Chapter 10"? If my adventure has a displacer beast encounter, can I not have an illustration of a displacer beast because that "refers to" the core artwork of what a displacer beast looks like?
5.5f. As others have pointed out, this means you can't incorporate GSL content into another product unless that product also follows the GSL. So you couldn't sell a box set of generic minis that also contained a GSL adventure, even if that adventure doesn't use those minis.
6.1. So you can convert a 3e/OGL product to 4e/GSL, but once you do so you have to stop producing print copies of the OGL version and stop selling downloadable copies of the OGL version. Basically, you burn that bridge behind you, you can't sell a similar product to the 3e market and the 4e market. Which means that if you convert the Most Popular OGL PDF Ever to 4e and it sells zero copies of its 4e incarnation, you're screwed, you can't go back and sell the old version ever again.
6.2. The funny repercussion of this is that it doesn't merely refer to titles of _your_ GSL books ... if Green Ronin publishes a GSL "Complete Book of Halflings," no other company using the GSL can publish an OGL "Complete Book of Halflings." And as the definition of "same or similar title" is really vague, they might not be able to publish _any_ OGL book called "The Complete Book of (whatever)" because Wizards' lawyers could decide the name is "similar" to the Green Ronin GSL book. (This hyperbole really depends on whether the definition of "licensed product" in section 3 refers to _all_ products published under the GSL or just products from that particular publisher.)
7. This is pretty much a continuation of the "decency clause" that was in the revised d20 STL.
7a. Which could mean that a product describing a torture chamber from the Inqusition is not allowed.
7b. Which could mean that you can't say the villain in your adventure is a rapist or a child molester. You can say he's a murderer or a druglord, though.
7c. Because you can't present one nationality as superior to another, you can't have a scenario where the Americans are the "good guys" in World War 2, coming to help defeat the evil German Nazis, even if it takes place in an alternate dimension or due to a time travel accident.
This also means you can't depict jihadist Muslim terrorists as inferior to anyone else or in a way that promotes disrespect (they're a political and religious group). Same for Fred Phelps's church. Or the KKK. Or the pro-slavery Confederate States of America. Or any nutty Christian who decides to bomb an abortion clinic. This section is pretty much the "don't say anything that could get Wizards sued" clause, except by policing GSL products so much they opened the door to make themselves liable in the first place (the OGL precludes all responsibility because there is no supervision or oversight).