One idea from Creativity Inc. was with me most of the time I was reading Mastery. Notice advices that are general and make sense the moment you hear them. There is a good chance that they are useless.
I have also heard a term TBU which means True, But Useless that is connected somehow to being more critical about general ideas and advices.
In my opinion, there is a reason why I was thinking about them so much while reading. Stories of master are really interesting and most of them are inspiring but advises that you get sound good but it is really hard to decide what to do with them.
Book is strongly influenced by 10k hours of deliberate practice rule, but I am not so sure if it really is true.
There is an interesting article that can give you more context how this 10k hours rule is just average and you can learn things faster or longer http://www.slate.com/articles/health_and_science/science/2014/09/malcolm_gladwell_s_10_000_hour_rule_for_deliberate_practice_is_wrong_genes.single.html. On the other hand, I have found also critic of the slate article: https://computinged.wordpress.com/2014/10/13/the-10k-rule-stands-deliberate-practice-leads-to-expertise-and-teaching-can-trump-genetics/ that tries to emphasise teaching over genetics.
No matter how you look at it, there are things we like more. Lessons at school we prefer. This might be math, language or music, but we all have some kind of preference. Maybe it is wise to focus on the things that probably would take us less time to master than the one that are really hard?
There is a strong preference of the author for an idea of Universal Man, man of a renaissance that masters various skills such as Leonardo da Vinci who was both scientist and artist. On the other hand, fact is that it is getting harder and harder to accomplish anything substantial in sciences. Nobel prize winners are getting older with time. Most obvious reason for that you have to learn much more to get to the frontier of research. How in this context can you master additional sciences when the first one takes you 20-30 years?
On the other hand from Learning how to Learn I know the idea of chunking. Basically the more you know, the easier it is to learn new things on top of that. Having vast interests also helps our brain create new connections. Learning to play a music instrument or new foreign language can help you with your research in your main field.
On the other hand, I found http://abetterlife.quora.com/If-you-want-to-follow-your-dreams-you-have-to-say-no-to-all-the-alternatives really inspiring and worth thinking about. Main idea is that if we have many interests then we are going to be shaped by circumstances and not by us. However, even Oliver Emberton agrees that you have different contexts in your life. For example exercising is one of those things that you should not ignore no matter how much you should/want to focus on one thing. Another such skill for me would be English language. No matter how much I would want to focus only on programming, for example, there is only so much, technical expertise alone can give you.
It took me a really long time to finish Mastery. In the meantime, I have finished Coursera: Learning how to Learn and read a lot of articles in Pocket. All of those ideas are interconnected now so it is hard for me to talk only about one thing.
So shortly about the book itself. Language and style are rather boring. Stories are, of course, the best part of the book. Some of them are inspiring, some of them are more like warnings what not to do.