I usually DM D&D all of the time and I very rarely get to play let alone play in a campaign. I am currently running my own game in a low magic world of sorts (compared to normal D&D). I couldn't publish said world due to multiple IP violations (FR, Eberron, Golarion, several 4th ed things borrowed) but anyway I sometimes do put restrictions in should I want to run a game.
Anyway my main restrictions which is almost universal.
1. No Samurai, Ninja, Wu Jen etc. Samurai= fighter, Ninja= Rogue (or multclass rogue), Wu-Jen= wizard. Restriction would go away in a Sppelljammer setting. There is no equivilent of China/Japan on my world.
Current game of 3.XYZ rstrictions.
1. You cannot start as a primary spellcaster (Sorcerer being an exception) and you can only multiclass into them at level 3. Spell casters are rare and the Red Mantis assassins hunt them down and kill them. Artificers, Bards, Duskblades, Alchemists, Magus etc are fine. Restriction applies to classes with level 9 spells. The assassins are kind of like the Sith in Star Wars.
2. Divine casters are even more rare. in addition to restriction 1 they have to find a divine power source. The gods are dead/absent and the BBEG race (Serpent folk/Yuan Ti) can somehow cast divine spells.
3. Wealth by level guidelines are gone along with most item creation feats. One can't buy magic items easily apart from ones bards, alchemists etc can make.
With the 2nd rule I told them that if they want to be a divine caster I will work it into the story (Luke becomes a Jedi after fidning Ben).
Obviously I would not put something like this in a core rule book (expansion book/camapign setting sure).
Where would one draw the line for the DM. At the end of the day they can do what they like but what in D&DN do you think counts as a restriction and is that restriction a good or bad one (PC's can't fly at level 1, PCs crafting magic items etc).