Is it legal and ethical to post my notes of a conference I am attending on my website?

Firstly, you will need to ensure that content from the conference is allowed to be shared - I have attended conferences were we were explicitly told not to share the content as much of it was pre-publication.

Secondly, and very importantly, if it is allowed and you do take notes, you should provide proper attribution - referencing the author/speaker and the conference proceedings.

If you are not sure of the legalities of reproducing your notes - ask and receive explicit advice.

This may be field-specific, my answer refers to public (in a way that attendees do not need any security clearance or something) computer science conferences:

I do not see any issues with posting these notes. In this day and age, commenting on whatever talk is currently being held live from within the talk on Twitter and Co. is becoming commonplace.

I have been to several conferences whose organizers specifically recommended a particular Twitter tag so the comments from the audience members could be quickly found online, and once, there was even a projector set up that would display any comments posted tagged with the conference tag throughout the run of the conference. Likewise, photos from the conference presentations have sometimes been posted by the conference organizers and others.

One of the nice features of this is that attribution comes almost for free - the conference tag already points out the event, and the time at which live comments are posted indicates what talk is currently being held. To make sure, you can always include a small note about the paper number or title, but that's already sufficient to unambiguously identify the paper and the authors based on the conference schedule.

Lastly, even though you may want to express certain opinions about the talks, in most cases it is generally good to be respectful toward the authors. Nothing is won by publicly humiliating them in any way; if you have objections against their presentations, make them specific and tangible. Also, make sure you are aware whether you are referring to an actual shortcoming of the underlying research, or just to a presentation issue due to limited presentation time. If you think the issues are serious, you might even try and contact the author for a clarification, and then integrate that additional knowledge when writing or updating your note. The only time at a conference when I saw participants write somewhat respectless Tweets on a talk (and thought they were rather appropriate) was when a business person literally flooded the audience with buzzwords (yes, in a buzzword bingo-enabling way) and thus delivered zero useful contents. That is definitely an absolute exception in conference talks.

The question of whether it is legal and ethical to take notes intersects with a number of different areas, each of which needs to be considered:

1. Expectation of Privacy: Are you going to be exposing the speaker to a level of publicity inappropriate with their reasonable expectation of privacy?
2. Copyright: Are you going to be violating copyright on the speaker's intellectual property?
3. Non-Disclosure Agreements: Are you going to be violating a non-disclosure agreement that you or your institution has signed? Such agreements are most likely to be the case when commercial secrets or non-filed patentable material is involved.
4. Security classification, arms control treaties, export restrictions: Certain data and technologies are subject to special regulation due to their potential to be used for harm.

For a typical open-registration scientific conference, you have no need to worry about non-disclosure agreements or controlled information. If you are in a situation where those are applicable, you will most certainly be told in no uncertain terms, and will probably have had to sign specific paperwork to take responsibility for this fact. These issues, though, are the ones that can cause real trouble if you screw them up. No open-registration scientific conference should ever have them apply, however.

That leaves copyright and privacy. Copyright isn't a problem if you're taking notes: that's your highly lossy paraphrasing of their material, and as long as you don't try to pass it off as your own work, it's both legal and ethical. If you were recording audio or video, that would be a different question, as well as potentially problematic for privacy.

As for privacy: if somebody has signed up to present their work in an unrestricted meeting, there is no expectation of privacy for the work. As for the person themselves, scientists are semi-public figures: as long as you are commenting on the public and professional aspects of the person, that is certainly acceptable (i.e., it's OK to talk about a scandal about a retraction, but at least ethically problematic to talk about rumors that they were raised by drug-addicted wolves).

Beyond that, it's common courtesy to make sure you get names and attributions right, as well as keeping you from embarrassing yourself. But your personal notes aren't a record with any special standing, and as long as you keep from actively slandering the speakers you should be fine. You still might cause trouble for a speaker who screws up and inappropriately discloses nuclear weapons secrets on pre-patent IP, but that's more their problem than yours...