So fun fact, you are no longer *literally* invisible with this spell, as the new invisible condition does not directly state that you are now invisible, and WotC did not update things like this spell accordingly. 

You are however *literally* concealed, and effectively as well hidden as someone who actually tries to hide, rolls stealth etc since there is now no hidden condition and the invisible condition is used for both.

You are literally visible with this spell.  It provides no evidence that it provides any effect that makes you 'impossible to see without the aid of magic or a special sense', as the 2014 did.   Look at it this way.  Change the name of the invisibility condition to something else, lets just keep it simple and say 'buffed'. And let's change the spell to 'Get Buffed'.

Get Buffed
Concentration
Level
2nd
Casting Time
1 Action
Range/Area
Touch
Components
V, S, M *
Duration
Concentration 1 Hour
School
Illusion
Attack/Save
None
Damage/Effect
Buffed

A creature you touch has the Buffed condition until the spell ends. The spell ends early immediately after the target makes an attack roll, deals damage, or casts a spell.

Using a Higher-Level Spell Slot. You can target one additional creature for each spell slot level above 2.
* - (an eyelash in gum arabic)

Spell Tags: Buff

Available For: Bard Sorcerer Warlock Wizard Bard Sorcerer Warlock Wizard Trickery Domain


Buffed [Condition]

While you have the Buffed condition, you experience the following effects.

Surprise. If you’re Buffed when you roll Initiative, you have Advantage on the roll.

Concealed. You aren’t affected by any effect that requires its target to be seen unless the effect’s creator can somehow see you. Any equipment you are wearing or carrying is also concealed.

Attacks Affected. Attack rolls against you have Disadvantage, and your attack rolls have Advantage. If a creature can somehow see you, you don’t gain this benefit against that creature.

Thusly... I'm in the middle of combat staring straight into the eyes of my enemy.  I cast 'Get Buffed',  I gain the 'Buffed' condition.  The first effect of being 'buffed' has no bearing, since initiative has already been rolled(in combat).  Me and my equipment are concealed though great.  Now I can't be affected by any effect that requires its target to be seen, unless the the effects creator can somehow see me.  My attacks are affected and I get advantage on attack my attack rolls, and attacks against me have disadvantage, again, unless that creature can see me.  Awesome, I love this so far.  But wait, my enemy can see still see me.  I'm right in front of him, and I've taken no action or gained any effect thus far that states otherwise. So I've just cast a spell, and gained  no benefit from these effects yet.  This is why as written,  the rules are confusing, and the spell is useless.     

좀 이상하긴 함. 은신 규칙에 투명 상태를 넣었으니 여기엔 마법적으로 투명해지니 뭐니를 넣어야하지 않았을까

It's kind of weird, because they put invisibility condition in the hide rules, so I think they should put something like magically blahblah here.

Yeah BUT...
You are right that, there is nothing that litteraly claims you are invisible. Instead this: "You aren’t affected by any effect that requires its target to be seen". It means that you can´t be seen... Isn´t it the point of invisiblity, kind of, that you can´t be seen?

This is simply trying to be dishonest/bash DnD.

They are usig the LITTERAL dictionary definition.

Also

The invisible condition is written in a way that hopes the players (DM included) use common sense.

Did you gain the invisible condition via mudane means, (eg hiding under a table or behind the curtain) Then you can be SEEN via mudane means. (eyeballs), Magical means to see a creature invisible this way isn't mentioned because it's given that if your not obscured and viewable by normal eyeballs the magical equivilent will see you anyhow.

Did you gain the invisible condition via magical means (aka the invisible spell) Then you can be SEEN via magical/special senses only. (Detect magic, truesight, tremorsense, see invisibility etc etc.

It is REALLY simple.

Would it have been better to mention this, kinda, but also it's not really needed.

Both invisible sub conditions can be DETECTED in similar ways. They might be LOCATED (not seen directly) because of footprints, walking on noisy ground or something like that. This gives the 'seeking party' a reason to investigate more.

As to why they didn't give the FULL PHAT description, have you SEEN the thickness of the books, if EVERY spell/ability was worded to the point of needlessness because of people being dishonest about interpritations and rule bending, the books would be double the thickness.

Each book already weighs 1+KG, and theres 3 books.

If you cast a spell with a long duration, like a minute or an hour, do you become visible again when you start or when you´re finished?

An if it´s in the beginning, can someone else turn you invisible after you start casting?

Quote from lnlyWizard10041 >>

If you cast a spell with a long duration, like a minute or an hour, do you become visible again when you start or when you´re finished?

An if it´s in the beginning, can someone else turn you invisible after you start casting?

Well, easy things first: since this is a concentration spell, if you had cast it on yourself, it instantly drops as soon as you start casting another spell that requires concentration (including any spell with a casting time of more than 1 action, which requires you to maintain concentration during casting). 

The more interesting question is when it would drop if it had been cast on you by someone else. As far as I can tell, there isn't any specific guidance on this in the Player's Handbook, but generally a spell isn't considered "cast" until the casting time is over (e.g., that's when the spell slot gets expended) so I think one could argue it doesn't break invisibility until you're actually done casting it.