So, the Youtube application finally entered the E-Shop after years of begging.
It provides you the Mobile smart phone version of You Tube with Videos accessible from your 3DS.
Now, It is notable that various Features of the Mobile version don't appear on the 3DS version.

Features:
- Video playback in 144p/240p (I'm not quite sure. The quality looks too good on my PC to compare)
- Possibility to Log in with your account & subscribe to channels

What you can't do:
- Watch 3D videos in 3D
- Post PM's or comments (however you can answer)
- Upload Videos
- Edit Video data (descriptions etc.)
- Enjoy all the good ads you would have on almost every other version of You Tube
- Watch Playlist Videos automatically
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In my opinion, I have to admit, the Applications isn't great but on the other hand I didn't expect it to be great.
Many people seem to be disappointed by the fact that it doesn't support 3D videos but I don't understand why.
I'm a You Tube junkie and I haven't seen many 3D videos on You Tube anyways. I mean there are some but thats the minority of the minority.
Since the 3D feature was added to You Tube I only found 4 or less Videos capable of 3D playback.
Not to mention I'm not a big fan of 3D gimmicks since I think that too many people focus them self on these 3D moments instead of the actual game or movie. I remember in some early 3D movies & Games I was like "Damn, look at all this wasted budged!" because there were many 3D scenes that didn't add anything to the actual movie/game.
The Video Quality is okayish.
HD rendered Videos tend to have better screen quality than the 480/360p rendered videos but either are watchable & enjoyable.
The Video Quality problem is something that has to do with the 3DS Hardware since the 3DS is only 240p so its really not a relevant problem for me.
Yet still, many complain because E-Shop & Nintendo Video (does somebody still use it?) even DPG on DS have better quality but you have to keep in mind that these Videos are rendered directly for the 3DS.
I would say its a nice addition to the free2use 3DS Apps. People with high expectations will be disappointed.