Technically, the America Invents Act requires the USPTO to accept and review submissions of prior art from the public. Other things which might invalidate a patent application, such as obviousness or lack of usefulness, are not covered by this law.
That said, (and this is a really important point so pay attention), there is no requirement that submissions of potential prior art necessarily be very good, and there is no limitation on what can be included in such a submission.
So, for example, if someone was trying to patent something obvious, you could submit some potential prior art (which might or might not be valid) and also add an explanation for why you thought that the patent was really obvious, and you would have a reasonable degree of certainty that the patent examiner would be aware of that. If you made a good case for obviousness, that patent examiner could certainly take it into account in deciding how to act on the patent application.
So, essentially, as long as there is a fig-leaf of prior art in the submission, you can sneak in a payload of obviousness with a high probability that the examiner will become aware of it.
THUS, you should certainly document cases of obviousness as meticulously as possible on AskPatents.com, but look for prior art, too (which should exist if something is truly obvious).
Also please bear in mind that in the world of patents, "obvious" has a somewhat technical meaning and there are specific ways that the USPTO uses to determine obviousness.