Implementing strict race selection.

[Celebrim]

It is entirely appropriate and correct for a DM to have strict guidelines as to what races and classes are appropriate to a campaign. While there are some sorts of campaigns that have a vaguely science fiction vibe, which benefit from a Team of Aliens theme and where racial novelty is a good thing, if you aren't running that sort of campaign (more Avengers and less Guardians of the Galaxy, more Tolkien and less Stars Wars cantina) then you should have a list of automatically approved races and classes to support your campaigns atmosphere.

For my current campaign the approved list of races is:

Sidhe (homebrew)
Changling (homebrew)
Pixie (homebrew)
Goblin
Hobgoblin
Half-Goblin
Elf
Human
Half-Elf
Drawf
Orine (homebrew)
Idreth (homebrew)

The approved list of classes is:
Akashic (homebrew based off Monte Cook's class of the same name)
Bard
Champion (homebrew based of Green Ronin's Holy Warrior/Unholy Warrior classes)
Cleric
Explorer (homebrew)
Fanatic (homebew, based off the Barbarian with ideas from Monte Cook's Oathsworn)
Feyblood (homebrew)
Fighter
Hunter (homebrew, based of the Ranger)
Fighter
Rogue
Shaman (homebrew, based off the Green Ronin shaman as a replacement for the Druid)
Sorcerer
Wizard

As a player, I'd be far more interested in someone's campaign if it had a rule like, "No humans", "No traditional fantasy races", "Only homebrew races", or some such than I would be in a game where kitchen sink was default in a thoughtless manner with an equally unimaginative Ren-Fair quasi-Tolkien setting as opposed to every race being available because we are going to play Planescape or Spelljammer pirates among the moons of an inhabited gas giant. Such restrictions would suggest to me a thoughtful and imaginative DM likely to run a clever and exciting game.

 

[Winterthorn]

I am a big fan of restrictions on races and classes available - if the restrictions make sense relative to the campaign setting. Using the particulars of a setting (e.g. ecological, historical, mythological, and/or cultural considerations) as a basis for some limits can cast a strong and interesting flavour to a game. Adding one or more new races or classes can fit with setting conditions as well. What do you enjoy? What would be creative? What would be a cool challenge?

Sometimes one can exclude a race, but use the published mechanics with a small tweak to two, and rename it, for a "new" race.

Another strategy, especially for club games and mini adventures, is to establish that race-class restrictions apply only to NPCs, and allow player's to "colour outside the lines" if they present a creative combo with an interesting background -- heroes are often exceptional individuals in some way, and they frequently arrive to the campaign stage from strange, serendipitous circumstances.

YMMV