First off - I don't think there is an obvious or right answer to the problem, or the airlines would all implement it. But here are my feelings:

I had a close friend who I travelled with once for business. He was a large guy, easily overflowing into my seat. I sat next to him on a two hour flight, and it was semi-uncomfortable. More on that flight later, but he didn't like being the size he was. He hated his weight.

I watched the same guy do the walk of shame at six-flags because he couldn't fit into the seat on one of the rides. I watched them try in vain to get the bar to latch, and it just wouldn't close, even with two employees pushing on it. This was in front of several coworkers. He later told me that when he flies, he is almost always treated poorly by those next to him. He had one flight where upon arrival to the aisle, the guy assigned to the seat next to him said "Oh [heck] no" and immediately turned around to find an attendant to change his seat.

Now back to that flight. He sat the entire flight with his arms folded and his legs close together, taking as little space as he could. He was respectful and kind to anyone he interacted with. It's just how he is as a person. Although it made the flight less comfortable, I made it to my destination just fine, and we had some good conversations along the way. As a side note, this same guy has since lost the vast majority of the weight and is much happier.

So my thoughts on the topic are pretty simple:

1) If you are overweight to the point that you will encroach on the seats around you, buy two seats if you are able. If you can't for some reason, be it financial or simply lack of seats available, then be as nice and accommodating as you can.
2) If you buy a coach seat, and get stuck next to a guy that takes up some of your space, be nice and kind, and deal with it for the small period of your life that it impacts. Maybe strike up a conversation and get to know the person, or just sit there and deal with it in peace. However, making it clear you're unhappy with the situation is petty and childish.
3) If you want a great flight experience, pay for first class. Or, you can be a jerk and make someone feel horrible while you enforce your rights to the airspace and seat space you paid for. Heck, if you're lucky, that overweight person will be a jerk too, so you can feel better about treating them poorly.

Basically, don't be a donkey to people and deal with what is truly one of the most first-world problems in an adult way.