Regarding the term "obverse", typically the obverse depicts the head (such as a monarch) or the principal design (such as the object of a commemoration). It often carries the date as well. But the obverse being "heads" is not the rule. Civil war storecards, for example, designate the merchant's descriptive information, with the reverse being whatever ornamental device is desired. The listing of reverse dies in the Fuld reference on storecards pictures numerous liberty heads, indian heads, and portraits of famous people like George Washington.

Islamic coins present their own difficulties with determining the obverse side. For the medieval coins, there are usually only legends on both sides. The attached article gives some insight into those problems.

Summary p190 second paragraph:
'The only solution is for scholars to define obverse and reverse in terms of the priorities they consider most important and then be consistent in their own work.'

Stephen Album in his 'Checklist of Islamic Coins' (3rd ed.) takes about half a page (p15) defining the obverse of a coin.