Is a wall an object? Let's say there is a stone wall 20' high and 200' long. It's filled with hundreds of glyphs. Would one casting of Dspel Magic wipe them all out?

Quote from Itap88 >>

It's known how this affects spells. But what about magical effects that have a non-instantenous duration but are specifically not spells? Say, a druid is under the effect of wildshape or a twilight cleric is under the effect of steps of night. What if such effect is targeted?

Magical effects that aren't spells aren't affected by Dispel Magic.

It's important to keep in mind that the description, not the name, of a spell is what defines what the spell can do.

Quote from Damonies >>

Tried to use this to destroy a construct. Did not go well. 

 If a construct has some weakness to anti-magic it is usually just action restriction and not dismantling.

Quote from Caladrel >>

Quote from Lazuli1925 >>

Could this spell work on a permanent version of major image?

 Major Image states "At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the spell lasts until dispelled, without requiring your concentration." 

This explicitly states that it can be dispelled, which includes Dispel Magic. Most permanent effects (all if I'm not mistaken) have a duration of either Instantaneous, in which case they can't be dispelled, or Until Dispelled, such as the upcasting of Major Image or True Polymorph after an hour of concentration.

Could you give an example of a spell with a duration of Instantaneous that has a long-lasting spell?

Quote from MarsExplorer >>

Quote from Caladrel >>

Quote from Lazuli1925 >>

Could this spell work on a permanent version of major image?

 Major Image states "At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the spell lasts until dispelled, without requiring your concentration." 

This explicitly states that it can be dispelled, which includes Dispel Magic. Most permanent effects (all if I'm not mistaken) have a duration of either Instantaneous, in which case they can't be dispelled, or Until Dispelled, such as the upcasting of Major Image or True Polymorph after an hour of concentration.

Could you give an example of a spell with a duration of Instantaneous that has a long-lasting spell?

A good example is animate dead, where the spell ends instantaneously but the effect of the spell (i.e., animating a dead body) can last much longer.

DC 19 spellcasting check and you kill someone's wish. That's crazy. 

Quote from DVision >>

Does having proficiency with arcana help the DC?

It does not, from the source I looked into.

 You have proficiency with your spellcasting ability.