Well, there are two different things you are trying to solve here.
1) Ease of use
2) Not having all transactions at the same mixin level
I don't think pushing the burden of 2 on to the average user is the way to solve it. Its a highly technical detail that 95% of users (or more) are not going to understand or want to think about, and it will constantly make them wonder if they are "doing the right thing" or if their transaction is secure enough. Could even discourage use.
The average user is completely ignorant and scared of technology. They know how to turn on their PCs, click the email icons, and that's about it. Thats not an insult to them. They are experts at their day job being doctors, lawyers, etc... but they just don't have any desire or understanding of tech in general. Given that the GUI wallet's purpose is to remove the complexity and make Monero accessible to the masses (if that's possible), then I would really focus on removing any "cognitive load" on the user. Don't force them to think about things that they can't possible understand anyway.
Perhaps a good solution would be to automatically randomize the mix in value between an upper and lower bound and not expose it at all to the end user. Advanced users that want to specifiy a certain value can do so in their preferences or an advanced options screen.
Don't you think this is a knowledge transfer / user education thing? I mean, after the first start wizard new users will see the tutorial screen (I'll show that off when completed), so before they see the privacy slider in the natural course of things they'll learn what it is.
Not showing the privacy slider effectively locks 90% of future transactions in at a single mixin (or even a random selection from a group), so this is something we need to keep loose so that the anonymityset isn't slightly reduced by a fixed mixin level.
