For most classes, yes it is more limiting. Warlock can take "Eldritch Sight" Invocation on level 2 and gain the ability to cast Detect Magic at will without spending a spell slot.

The "R" in the little book there means it can be cast as a ritual. Which, kind of rains on the parade of making this not a 0-level spell, but it does take extra time if they don't use a spell slot.

Colors of Magic

     Detect Magic or other abilities that allows seeing magic often shows such a presence through an aura emitting from the magical object. Without a more direct Identify spell, that grants the caster full knowledge of the magical item, someone seeing the aura of magic will see a system of varying colors that denotes the school of magic the magic is based on. Additionally, the aura may have different characteristics granting the DM the ability to give their description a little more content such as a flame like movement; a dense hue around the object that fades after an inch or two; a crackling electrically surging aura[possibly to denote intense power];  a slow and transparent molten aura that ends cleanly and without fading;  an almost reflective colored shimmer that reveals itself when inspected; and so on.

The different schools of magic are as follows:

Abjuration, Conjuration, Divination, Enchantment, Evocation, Illusion, Necromancy and Transmutation

     Often when a spell-caster uses magic the color of the school of magic is visible temporarily by those unable to see magic auras. This is because the potential energy of the spell is gathering and the density and concentration of magical energy becomes visible.

This said it should already be almost self evident which colors represent which school of class. To clarify:

     Abjuration: Gold(yellow)
     Conjuration: Green
     Divination: White
     Enchantment: Silver(Teal)
     Evocation: Red
     Illusion: *Rainbow
     Necromancy: Black
     Transmutation: Blue
*Illusion spells contain a myriad of distinct colors forming a shifting rainbow pattern. Once the spell is cast the colors may give way to a single primary/dominant color.

There are additional colors to consider:

     Grae: This color is almost pale and appears on more ancient magic that denotes something to do with Transmutation, Enchantment and/or Divination. This color generally indicates good will (But not always).
     Purple: This color represents a more dangerous magical object relating to Necromancy, Evocation and/or Transmutation. While Grae and Brown may also be equally dangerous, a magic item with a purple aura often indicates the creator had more of a sinister nature (But not always).
     Brown: This color varies in strength depending on how much power is with the magical object and also appears on ancient magic but refers to something beyond the combinations used to emit Grae or Purple auras. These magical items are often created by some ancient neutral party (But not always).

Situations of Customizing/Changing Magic colors:

     On occasion a character within a class may exude their own magical aura. The terms in which the aura is colored is based on one of two aspects.
When a spell-caster uses magic, the colors of the aura will waver between the color of the school of magic in which the spell is from or the spell-caster them self. The innate magical talent within a spell-caster, once grown, may emit its own unique color. This is because the body has been primed and saturated with enough magic that it in itself is considered magically inclined. In such situations the spell's aura will be either the color of the spell's school or the color of the individuals own magic aura to denote whether the true source of power from the spell is from the precision of execution(school) or the energy placed into the spell(spell-caster). For example, a cleric with a white aura casting a transmutation spell will show a blue aura if the spell's power is from the character mastery in casting the spell or white if the spell was cast correctly but not perfectly(92%) and draws its power more from the spell-casters own energy repository.

     Additionally the aura may be a combination of the two colors if the spell is cast with great power. One example is casting a spell at a higher level but may also happen when the character is situationally/emotionally stimulated.

Why can't Warlocks get Detect Magic? It's available to every other caster class.

Sure, there's an Eldritch Invocation for that, but you can swap your spells out at level up.

Also ritual casting...

Casting Spellword is:

Eldritchium Arcanicus

Warlocks can swap an invocation every level up too.

Also Detect Magic does not let you see invisible objects but does let you know that something magical is in the area aka 5 foot square?

thus you would get disadvantage but you would know something is in that area

Potions are a bit murkier, so house rules all the way. Where is the fun in life if every poison potion comes up as necrotic, and every potion that heals comes up as evocation? Unless very kindly, at our gaming table a potion will have a transmutation aura, and colour,  consistency, transparency, odour, etc. that are purely descriptive. Until one can get them back to lab for study, taking a (possibly lethal) swig is usually the only way to know for sure exactly what that steaming, green fluid is likely to do. Although, the same recipe by the same cook should generate potions with a similar description.

*Items with effects, powers, etc complicate this. 

The colour of a school is discussed in other posts in this thread.

JGoka and UncleKyle, your responses are great!  Are these your homebrew? If not, could you please cite your references.  Thank you.

"see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any."

"The spell can penetrate most barriers,"

I have 2 questions. DMing the first campaign I've ever played, trying to get as much accurate info as possible.

1. "See a faint aura around any..." Does that mean all, or does that mean 1 of your choice? I've been applying it as all, but if that's not the case then I will explain that prior to the next session and play this better moving forward.

2. Doesn't penetrating most barriers kind of contradict "visible?" If something magical is on the other side of a 1-foot wood wall, could you detect that? If not, what's the point? And if so, then what does "visible" mean here?

I can only answer your second question.

You can sense that there is magic from behind a 1-foot wall, but you cannot discern which school that magic relates to. For example, a character can sense that there is magic behind a door, but not what kind of magic. To se the aura around an object the object has to be visible to the character.

Look up ritual spells

So, can someone see through a magic illusion that I cast if they cast detect magic? Making sure so that I can play an illusionist properly. Also because some guy in my group says that he’ll be able to see through my illusions 100% of the time because of some High-Elf detect magic cantrip, and plots to ruin the illusions. 

Detect Magic is a Ritual Spell.  It will be spammed.  Just put in a random encounter check for each 10 mins of casting.

no you can just ritual cast though

Quote from Schanez >>

For most classes, yes it is more limiting. Warlock can take "Eldritch Sight" Invocation on level 2 and gain the ability to cast Detect Magic at will without spending a spell slot.

 I just got warlock lv. 2 and I'm looking at getting this.  Seems like it's more for exploration vice combat, but that's ok.  Right now I'm leaning towards Devil Sight and Eldritch Sight for my Paladin 6/Warlock 2.  It'll just add to my senses...

Quote from FritoSan >>

As a GM for another system considering switching to DnD 5e, I'm super stoked that Detect Magic is not a Level 0 spell. This should prevent spamming and force the player to really justify (to his/herself) risking the spell slot use.

 You are exactly right, I cannot IMAGINE where I would ever have fun spamming detect magic, walking the halls of cryptic dungeons. Do be careful though, your players can ritual cast Detect Magic and for an increased casting time, can cast it without expending a spell slot. Make sure to punish those who still spam and ambush them w/ monsters while they're waiting all that time to cast ( ͡~ ͜ʖ ͡°)

Until you have a wizards and ritual cast it ever 5 mins