Knowing the basics is good, but going over the rules in some more advanced grammar book can help you even more. In "A Comprehensive Grammar of the English Language," by Quirk et al (you can easily find it on the Internet), you can read under item 5.37, page 273, the following to answer your doubt about "a really good place" being used to refer to a place you already know:
Nonreferring uses of the indefinite article
5.37. The indefinite article is strongly associated with the complement function in a clause, or more generally with noun phrases in a copular relationship [...] Here it has a descriptive role (similar to that of predicative adjectives), rather than a referring role:
- Paganini was a great violinist.
- My daughter is training as a radiologist.
- We found Lisbon (to be) a delightful city.
- What a miserable day (it is)!
Wheras the indefinite article is required in the previous examples, there is vacillation in the following cases [...]:
- her duties as (a) hostess
- my appointment as (a) lecturer
- Jung as (a) thinker
Sometimes a/an is nonreferring in a stronger sense; it may not refer to anything in reality at all:
- Leonard wants to marry a princess who speaks five languages.
The answer to your specific doubt is that in "This is/It's a really good place" the indefinite article is required because it introduces the subject complement of the sentence. The answer to your more general question is, study the rules from more advanced books.