OSR: Oh Fuck It's A Wizard!

The GLOG uses levelless spells. It's very convenient. Wizards invest between 1 and 4 Magic Dice (MD, or [dice]) in their spells, varying the damage, duration, area, effects, and power. More [dice] in a spell invites more risk.

David Perry suggested

In a low-ish magic setting where MUs aren't flinging cantrips left and right, casting ANY spell that isn't subtle should cause a Reaction re-roll and/or Morale roll, and/or standing in stunned awe for a round. Including hirelings. Restrict it to creatures of HD of, or lower than, the spell level, and/or only if they haven't seen the spell before, if you like.

Here's how to apply this concept to the GLOG, using classic spells on the Orthodox Wizard spell list as examples. In my campaign, wizards are not particularly rare, especially near wars, but very few can cast with three or more Magic Dice. Morale tests and reaction rolls may not be required if witnesses have seen magic before.

Spells are also great excuse to test your powers of description.

Pedro Krüger Garcia

Cantrips

Most cantrips are so subtle even the caster may not know they are being used. They are soul-mutations, spells grafted onto the wizard's very essence. Unless the cantrip has an obvious effect, only a wizard with active Wizard Vision can see one being used.

fah-qeu

1 Magic Dice

The spell is subtle and unimpressive, cast silently and with a few small gestures. Unless hirelings or enemies are particularly suggestible no Morale checks are required. People will say, "Not much of a wizard."

Knock: a door creaks open as if shoved. A lock clicks and falls.
Light: the wizard blows on a crystal and it slowly begins to glow.
Magic Missile: a quiet dart of rippling magic arcs from the wizard's finger.
Fireball: a dull red marble, followed by a cough of flame

Tales from Earthsea

2 Magic Dice

The spell is obvious, cast with a spoken word, a clear gesture, and a distinct crackle of magic. Most war-wizards cast at this level. Hirelings and enemies must test Morale or be stunned for 1 round as they watch the fireworks. People will say, "Yup, that's a wizard." 

Knock: a door slams open as if kicked. A lock smoulders and crumbles.
Light: the wizard summons a glowing aura in one hand. The light pulses and ripples.
Magic Missile: a glowing bolt of light hisses through the air, leaving a trail of sparks and smoke.
Fireball: a glowing orange marble, followed by an ear-popping thump of flame.

Jonathan Strange and Mr. Norrell

3 Magic Dice

The spell is magnificent, cast with grand gestures, words of power, and secondary effects. The wizard's hair is whipped by spectral winds. Shadows lengthen and darken. For a few moments, the wizard seems to be more real than the rest of the world. Hirelings and enemies must test Morale as normal. People will say, "Oh fuck, it's a wizard!"

Knock: a door flies open as if hit by a battering ram. A lock disintegrates into a pile of smouldering ash. Unlocked doors, windows, or cabinets within sight of the wizard may also fly open. Contents of drawers may fountain into the air.
Light: the wizard summons a brilliant orb. Rays of scintillating light halo the object.
Magic Missile: a screaming arrow of rippling magic tears through the air, leaving a trail of glowing fog. 
Fireball: a yellow marble, covered in flickering fire, followed by a thunderclap of flame.

Harry Potter

4 Magic Dice

The spell is terrifying. Wizards who can cast at this level earn titles, "Archmage", "Dread", "Magister", "Shadow-Queen" from witnesses. They don't nessesarily earn tenure or rank in the feudal hierarchy, but they start to get a reputation. The wizard is outlined in magical radiance. They hover. Flames flicker, clouds boil, water freezes or rolls back. Time itself seems to stretch and bend. Hirelings and enemies must test Morale with a penalty. People will say, "AAAAH! AAAAAAAAAH!" or just point and stare.

Knock: a door flies off its hinges. A lock melts into slag or is simply unmade. Unlocked doors, windows, and cabinets burst. Drawers fly from their homes. In the distance, echoing,  doors can be heard slamming open.
Light: the wizard draws the light of sun or moon into the palm of their hand. Shadows flee. Creatures of darkness cower.
Magic Missile: a comet of magic and sparks shrieks through the air, sizzling and spitting.
Fireball: a white hot marble, radiating jets of fire, follow by an ear-shattering explosion of flame.

exellero

5+ Magic Dice

The spell is legendary. Few wizards can cast at this level; fewer still would dare. The risks are too great. Wield stave of power, pour molten occultum into your living brain, make a bargain with a powerful creature, or perform other dangerous rituals. Hirelings and enemies usually flee automatically, although they may regroup once the spell ends. People are usually too busy running or bowing to say anything.The spell leaves a permanent mark on the world. 

Everyone who sees it will remember it for the rest of their lives. It's unfortunate that their lives might not be very long.

Knock: every door, lock, drawer, box, and cask in the area explodes, sending shrapnel through the air. Philosophical concepts, secrets, and natural laws may also be unlocked.
Light: the wizard draws the presence of the sun into the world. Iron shimmers, as if drawn by invisible hands. 
Magic Missile: a fissure in reality, a thunderbolt of negative space, evaporating everything it touches.
Fireball: a purple orb, thrumming like a guitar string, followed by a silent faster-than-sound burst of purple flame and burning light.

Marc Simonetti

Other Spellcasting Classes

Paladins of the Word speak with the literal voice of the Authority. They cannot whisper a divine command. Bystanders tend to kneel, spontaneously obey the command, weep, pray, or flee.

Sorcerers spurn subtlety. What's the point of being a sorcerer if you can't show off?

Summoners are subtle by default. Some of their entities may provoke immediate Morale tests.