Combat capabilities: It is rather rare for a Magus to be able to combat a Servant, and when it happens, it usually is a weaker Servant.
Command Spells: As noted in the Servant rules, there are command spells for every Master. The Master is able to ask of anything from the Servant, so long as they have the Command spells. They are only allowed to defy the will of the Servant 3 times, afterwards the Servant is under no obligation to follow orders.
Making contracts: A Master is capable of making new contracts, but the Servant they make a contract with must
Magic: When it comes to a characters magical ability, I will be up to debate over that sort of thing. However, I won't allow a character to branch off too far.
http://typemoon.wikia.com/wiki/Magecraft
Look here for examples of magecraft people are usually capable of.
