Great question. Fred BC gave a fantastic answer, and I agree with everything he said. However, I'm still going to try to put my own thoughts into words.
Description of reality seems like a good definition of "fact," but it cannot be the only part of the definition, especially because so many things that are not strictly "descriptions of reality" are considered to be fact. For example, the statement. "I think Obama is doing a great job" is a fact. Whether or not what you think is true, it is objectively true that you think it. This doesn't fit strictly within the "description of reality" definition, as it's an objective descriptions of a subjective internal, unverifiable feeling. But then again, it all depends on how you define reality, so based on the definition of reality, this definition can still hold true.
An opinion, on the other hand, is arguable. If there is an argument to be made against it, then it is an opinion. But when I say argument, I don't just mean denial. I mean a legitimate disagreement with a legitimate basis. "Obama is a good president" is an opinion. I could counter it with an argument attacking his drone attack policy as morally unjust, and therefore he is evil. I could also make the argument that because he's black, he cannot possibly unite the country in any legitimate way. Neither argument is good, and one is outright offensive. But because these arguments, very simply, can be made, we must give the original statement that status of "opinion."
But then we run into a statement like "global warming exists and is currently occurring." That is a fact, yet are there not many who argue against it? Sort of. The arguments against global warming are hardly argument at all. Why? Because they are not based on, or represent an objective misrepresentation of, fact.
And that's ho we identify what an argument is and is not. For my purposes in this definition, an argument is, at its core, an interpretation of/conclusion drawn from solid fact.
Example: Neighborhood X has above average gun crime and above average gun ownership. Fact.
Argument: Gun ownership should be restricted in X to try to reduce the gun crime rate.
Alternate argument: Gun ownership should be expanded in X to allow people to protect themselves from the rampant gun violence.
Both arguments, both basis for wider opinions and opinions in-and-of-themselves, and most importantly, both based off of fact.
But with global warming?
Fact: The planet is getting warmer.
Fact: The planet goes through climate cycles.
Argument: This warming period is simply a natural warming cycle.
At first, it appears to be based off of fact. However, in reality it is simply a representation of fact. There is no third "fact" tying these two together. In reality another "fact," that the current warming cycle does not align in any way with normal warming cycles, shows that this "argument" is actually based off of a misunderstanding of fact, and is therefore not an "argument" at all.
So in order to judge something as an opinion or not, you must look at, "Is it arguable?" and "Are the arguments to be made legitimate fact-centric arguments?"
This sort of answers another question posed above as well. An argument serves sort of a as a mid-way between fact and opinion. An argument can be an opinion in-and-of-itself, but more often the word "opinion" implies the amalgamation of all arguments that an individual ascribes to on a particular subject to form the individual's holistic view of the subject. An opinion, then, is a tiny chunk of worldview. That is the main differentiation between argument and opinion (although, again, they do not block each other out, and one "thing" can be both argument and opinion):I can flipflop arguments all day long, as long as I have the fact on which they are based and the skills necessary to draw multiple logical conclusions. But even though I can feign it, I cannot flipflop an opinion as easily. Because it is a piece of how I view the world.
That is why it is easy to beat someone's arguments, but difficult to change their opinion.
So therefore, if an opinion is simply a chunk of worldview, then yes, an opinion must always be subjective. And the difference between subjective and objective? Objective shares the definition of "fact," but it also expands to statements that are basically never argued (at least that is the implication). "Total anarchy is bad" is an opinion, but it is one that can be considered basically objective. Subjective refers to all that exists within a person's head, including their interpretation, by the senses or otherwise, of the objective world around them.
As to whether they are good or bad, objectively? Well, to say so would 1) inherently be subjective, because I am only me, and not everyone and everything, and 2) be an opinion in-and-of-itself. Therefore this question is unanswerable by conventional means and is self-defeating.
This question is interesting: "If 99% of people think A and the 1% think B, is the 1 percent's opinion still an opinion, or is it just an unsound judgement?"
Referring to the anarchy example above, we can only call something "objective" once basically everyone is in complete, unarguable agreement on it. That means that opposition is negligible. 1% is not negligible (in my view), so this question doesn't directly apply to that example.
However, "opinion" and "unsound judgement" are not separate things, not entirely, because judgement has a large overlap with opinion II see judgement as largely the action aspect of opinion, the way it manifests itself). The 1% may have an unsound judgement/opinion, but that doesn't mean that it is not a judgement/opinion.
Furthermore, the 99%, or even the 99.9%, does not decide what is and is not sound. Logic and factual basis decide that.
As for how our opinions are related to knowledge, experience, and intelligence? Well the greater the knowledge, experience, and intelligence of a person in an area, the stronger, more thorough, and more well-developed their opinions, judgments, and arguments will be in that area (and any relating areas as well).
And yes, because it is based off of legitimate arguments, and can itself be argued through legitimate arguments, everything said in this response is an opinion.