Nice topic. I think there is a fine line between giving an opinion on topics and expressing what you understand the answer to something is, being open to correction. In the process of you giving your understanding of something, it gives more learned people an opportunity to correct you if you are incorrect. It is also very important to base your "understanding" or "opinion" on something so that when someone asks a question, you rely on something in your answer, whether it is the Quran, ahadith etc. or state where your understanding/opinion comes from.
I think most people these days rely on the internet for answers, and overall, this is a positive as the internet has a wealth of information out there. The problem is that not all websites are correct and have been based on a sound Islamic analysis on different issues. Hence, even if people base their opinions on websites they found on the net, they should at least disclaim this.
When you go to a forum thats about trading shares, you are always required to provide references for any "facts" that you say. In this way, this reduces rumours and incorrect information being circulated, which can be damaging to other traders that are not too familiar with all the 'facts'... same thing applies here.
Hence, in short, as much as possible, people should state or disclaim their source of understanding, even if thats a "I heard this as a kid". In that way, everyone else is in a better position to re-analyse and re-evaluate such views.
Allah has given all of us intellect and we all have the ability to try and understand different things within Islam, nay, we have the obligation to do so. However, that does not mean that unlearned people will necessarily have the means to make a sound conclusion based on all of the evidence available taking into account many factors. Hence, why people say leave the decision making to those with the authority to do so (i.e. issue fatwa's).
That doesnt mean they'll always get it right and if they dont, usually there are other learned people there to correct them based on sound reasoning...