I think Sejanus.Aelianus have written a lot of good points that I fully agree with, and I find it refreshing to read.
What I also think is that being good at something in many cases actually means "being good at knowing what will please others". And there are many factors that affects what pleasing other means - who they are, where the world is at, what has been accepted long ago as right or wrong etc. Also who you are, if you are known since before, what you did to get your result etc.
I think some of my own photos are crap, some are good and a few are great. But looking at the overall response I get on them I know that most people place them in the "bad" category. Does it mean I'm wrong? No, it just means that I either suck at seeing what other people considers to be good, or I chose to ignore it.
Then of course there are things that almost everyone agrees is bad, or everyone agrees is good etc. But those are often the extremes. In between, in the lovely gray area, it's a lot harder to define good/bad/right/wrong.
Same thing applies to music for example, painting etc. But it does not apply to very concrete things like running a pre-defined distance the fastest, I'd say.
If you want to hit it off professionally, I think "knowing what others will consider good" becomes extremely important. It's nothing strange really, it's what we all do every day more or less in our "normal" day jobs. If you don't care about taking it to that level but instead do it for fun, you can either choose to just do what you do, change what you do, or do what you do and find the people that likes the same thing if you are after praise.
I'm sure a lot of people doesn't agree with me, but this aspect of good/bad has always bugged me to an extent where I sometimes completely lose my temper just thinking about it. I find it so extremely annoying that sometimes awful products (be it art or whatever) gets praise and I just don't see how it can even be considered remotely interesting. But then again that is just my opinion on that specific piece. I think it is important to remember than when someone says what you do isn't good enough. It's just their opinion. Then after that you have to think about what you are after with what you're doing.