It always bothered me, kids, when they come to Hogwarts first, start learning via basic spell books and later on they learn advanced magic some of which is, as I consider, not that more challenging than that from basic spell books.
Students aren't allowed to study on their own, basically, and are not allowed to grab more challenging books from the library, right? Even if, as Hermione's example shows, they are allowed then not all magic comes easy. So, truly, for different magic different restriction exist. Hermione, how well she hadn't tried, couldn't effectively cast expecto patronum or other advanced magic. However, there is a nuance, whether she could find information about such spells, or whether she wasn't able to master them. Dumbledore can place an anti-apparation charm upon a castle, it's unlikely that Hermione can do it though, although she is a very skilled witch. On the other hand, Malfoy can easily cast Serpentsortia, although nothing is said about how difficult it is to cast it or to find out about it, or even why a second year knows it and how he found out about it.
Taking different magic, we see that some spells are easily studied and are simple to master, while others are not, but what makes the change? I am aware only of the restrictions to effectively cast the three unforgivable curses, and expect patronum.
The question is, what other spells are there with certain restrictions for effectiveness and what needs be done to master them?
P. S. So to say, why some wizards are able to cast some spells more effectively than others?
P. S. I would also like to know, whether it is because of the restriction that students aren't allowed to try and learn advanced magic or other kind of magic (non school program approved) on their own.
Of course, there are exceptions, such as Tom Riddle and Dumbledore, probably, Snape, Sirius, James, Lupin, Horace, and Grindevald probably.