First, the first amendment has absolutely no relevance here, since it is a restriction being placed on government, not on individuals. There is a chronic habit to misinterpret this as "I can say what I want, wherever I want, whenever, however", which is an interesting opinion, but one with no correlation with the legal definition. The 1st isn't intended to--and doesn't--defend the imagined right to express your views on other people's servers.

Next, your real life examples are a bit sensationalist. The difference is that Saddam was killing people for expressing opinions, whereas we want to downvote poorly expressed and undefended opinions. A more apt comparison would be the university setting, where challenging opinions will be accepted on the merits of their presenter's ability to defend them. The same applies to Galileo, who was persecuted for his theories (not, note opinions). I believe that we'd be overjoyed to have a Galileo posting here, since he would link to his mathematics and let us examine the feasibility of his theories for ourselves.

Finally, to address the meat of your argument against downvoting, I think you have misdiagnosed why people are downvoted. I rarely see people downvoted for their opinions, but frequently see people downvoted for failing to effectively defend or explain their opinions. Since we are trying to foster discussion, the actual opinion is usually less important than explaining how they have arrived at their opinion, and why their opinion is viable. Simply expressing an opinion is noise in a conversation often boils down to noise, and opinions on their own won't feed the starving mind: the meat of the argument lies in explaining opinions.

Thus, I believe that the downvote allows the community to exert a stronger preference for comments that add value. If you want to avoid being downvoted, express you opinion and then defend it.