Disclaimer: IANAA (I am not an American)
I thought one of the core characteristics of the USA was democracy and the idea that the country is ultimately ruled by the people. As such, there is nothing that excludes somebody from still loving their country but seeing how things are done somewhere else and wondering if it is a good idea.
Of course the USA has different priorities, but aren't those priorities set by the people? If 90% of USAians decide they no longer want to allow people to own guns in their country, does that make it any less USA? (Note, I'm not saying 90% do and I'm not saying guns should be outlawed - just making a point about democracy).
The great thing about democracy is that you have the ability to influence the country in which you live. The great thing about freedom (as you pointed out) is that if you disagree with the rest of your country people you can choose to leave (provided there is somewhere else that will have you).
I believe that as the world changes, so too must people. Representatives are elected to make decisions as to what the priorities should be. The hard part about government is that it seldom happens that people agree on every single issue. It doesn't matter who is elected to govern, both individually and as a collective, they will disagree on *something* with almost everybody in the country.
So to me, to say that there is no room for people to want things to be different is un-'American' - indeed, few politicians get elected on a platform of "I'm going to keep things exactly the same as they are" (as if that is even possible). Rather, most politicians get elected because they have a vision of how their country can be better and a plan for how that might be brought about. If you happen to disagree with choice that your fellow citizens have made, well, you either advocate for your position in hopes that it might be changed, or, as you said, you go somewhere else.