[A]ll of [the speaker’s] statements concerning the allegedly poor results of her surgery are protected opinion, because they do not imply the existence of undisclosed facts. Basically, she says she had the surgery, and she has the unfortunate conditions described. Also, in her opinion, they are the result of the surgery, which — also in her opinion — involved negligence on the part of [the doctor]. These are all the facts she adduces; she does not imply the existence of any undisclosed facts. The reader of the postings may decide for himself or herself whether the opinions should be accepted, or are an example of the logical fallacy known as post hoc ergo propter hoc.
But there’s a more detailed answer, which the “do not imply the existence of undisclosed facts” alludes to; here it is, from the Restatement (Second) of Torts, which sets forth the broadly accepted view on the subject among American courts (some paragraph breaks added):
There are two kinds of expression of opinion. The simple expression of opinion, or the pure type, occurs when the maker of the comment states the facts on which he bases his opinion of the plaintiff and then expresses a comment as to the plaintiff’s conduct, qualifications or character…. The opinion may be ostensibly in the form of a factual statement if it is clear from the context that the maker is not intending to assert another objective fact but only his personal comment on the facts which he has stated.The pure type of expression of opinion may also occur when the maker of the comment does not himself express the alleged facts on which he bases the expression of opinion. This happens when both parties to the communication know the facts or assume their existence and the comment is clearly based on those assumed facts and does not imply the existence of other facts in order to justify the comment. The assumption of the facts may come about because someone else has stated them or because they were assumed by both parties as a result of their notoriety or otherwise.The second kind of expression of opinion, or the mixed type, is one which, while an opinion in form or context, is apparently based on facts regarding the plaintiff or his conduct that have not been stated by the defendant or assumed to exist by the parties to the communication. Here the expression of the opinion gives rise to the inference that there are undisclosed facts that justify the forming of the opinion expressed by the defendant.To say of a person that he is a thief without explaining why, may, depending upon the circumstances, be found to imply the assertion that he has committed acts that come within the common connotation of thievery. To declare, without an indication of the basis for the conclusion, that a person is utterly devoid of moral principles may be found to imply the assertion that he has been guilty of conduct that would justify the reaching of that conclusion….A simple expression of opinion based on disclosed or assumed nondefamatory facts is not itself sufficient for an action of defamation, no matter how unjustified and unreasonable the opinion may be or how derogatory it is. But an expression of opinion that is not based on disclosed or assumed facts and therefore implies that there are undisclosed facts on which the opinion is based, is treated differently.The difference lies in the effect upon the recipient of the communication. In the first case, the communication itself indicates to him that there is no defamatory factual statement. In the second, it does not, and if the recipient draws the reasonable conclusion that the derogatory opinion expressed in the comment must have been based on undisclosed defamatory facts, the defendant is subject to liability….Illustration 3. A writes to B about his neighbor C: “I think he must be an alcoholic.” A jury might find that this was not just an expression of opinion but that it implied that A knew undisclosed facts that would justify this opinion.Illustration 4. A writes to B about his neighbor C: “He moved in six months ago. He works downtown, and I have seen him during that time only twice, in his backyard around 5:30 seated in a deck chair with a portable radio listening to a news broadcast, and with a drink in his hand. I think he must be an alcoholic.” The statement indicates the facts on which the expression of opinion was based and does not imply others. These facts are not defamatory and A is not liable for defamation.By way of recapitulation, the effect of the rule that there can be no recovery in defamation for a pure expression of opinion can be set forth by applying it to four fact patterns:(1) If the defendant bases his expression of a derogatory opinion of the plaintiff on his own statement of false and defamatory facts, he is subject to liability for the factual statement but not for the expression of opinion.(2) If the defendant bases his expression of a derogatory opinion of the plaintiff on his own statement of facts that are not defamatory, he is not subject to liability for the factual statement — nor for the expression of opinion, so long as it does not reasonably indicate an assertion of the existence of other, defamatory, facts that would justify the forming of the opinion. The same result is reached if the statement of facts is defamatory but the facts are true ….(3) If the defendant bases his expression of a derogatory opinion on the existence of “facts” that he does not state but that are assumed to be true by both parties to the communication, and if the communication does not give rise to the reasonable inference that it is also based on other facts that are defamatory, he is not subject to liability, whether the assumed facts are defamatory or not.(4) If the defendant expresses a derogatory opinion without disclosing the facts on which it is based, he is subject to liability if the comment creates the reasonable inference that the opinion is justified by the existence of unexpressed defamatory facts….
The court’s view, I take it, is that a reader would read the patient’s comment and think, “she’s saying that she was operated on by the surgeon, which presumably she knows for a fact, and that she now has certain symptoms, which presumably she knows for a fact, and now based on these disclosed facts she’s expressing the opinion that the surgery caused the symptoms, and that the doctor was negligent.” In this respect, the court views it like illustration 4, though with fewer facts asserted. I think that in context it’s a reasonable decision on the court’s part, though of course on such matters of textual interpretation people often disagree.