What is Good Content anyway?
I am fairly new, but I came here to deliver quality content.
Now, I know lots of people have different opinions of content, for me that means lots of deep-thought text.
But many people are only posting some pictures (which is OK if you want to see pictures, but why without any explanation where and when and why?). But what do you learn by just having a row of pictures? (Like here, just randomly picked from the newest posts: https://steemit.com/art/@arjunmartavela/the-art-of-flower-roses )
Others just put in a youtube video. Even if that is their own video, it is… you know, worse then youtube!!
I would not even consider that content. Why not writing one or two paragraphs at least, explaining what is in the video, so a potential viewer has not to waste his time on it?
Or preferably the content or a transcript of the video. After all its easier, faster and more flexible to read that then watch a clumsy video.
So what do you think is good content? What shall I do with those I think are not, like just a single picture or a video? Flag them?

No, you definetly shouldn't flag that.
What can you do? - Write better content )
Maybe even more important, vote for better content )
I agree with you @svamiva flagging would not be a good practice, just don't vote for it. People are bound to realize that content is important to add to photos, videos etc. Newbies may not quite get the platform yet and how to use the relevant tags. Give everyone a chance to learn how to be successful on Steemit.
Thanks for the clarification.
And yes, I vote. A lot. If there is quality. Yesterday was not much, I was surprised O.o
But it is also frustrating for a newbie to know that even 10 of his votes do not amount to even a single cent.
Even if your vote do not give much money, it's doing something even better - it attractes attention, doing it two ways.
One way is that it's keeping the post floating on active page, giving a chance for more people to see it.
The other way is, that it also attract attention of curation bots too.
If you check the list of most effective curators http://steemwhales.com/trending/?p=1&d=7&s=cr
you'll see most of top positions occupied by bots. Most of those bots are using very simple algorithm to vote for posts which already got something 5+ votes from accounts with rep score >40+ , but still having reward below 0.01$ ( details of course may vary )
Now, because those bots are so effective , others ( including whales ) are starting to follow the same trail...
I would recommend you to check out this post about curation
https://steemit.com/steemit/@shaka/i-care-about-voting-how-about-you
Sorry, maybe not so to the topic, but still one of the things necessary to do to make your content good and easy to be found is to post your content under the relevant tags..
Otherwise I do agree with you that posting just a youtube video is not OK. :(
I've not been on steemit long myself but every so often I go through the people I follow and double check I like the content and think its valuable, upvote/resteem.
I see post with just videos and pictures. Personally I think something someone has drawn or taking a picture of is fine content, to me they have put in some thought and time drawing or even taking the picture.
With videos I would like to see a paragraph or 2 explaining the video, sort of like a blurb/teaser.
I dont think I always get tags right but I try my best to properly tag my posts.
I agree with your thoughts about paintings and original photos, as well as about the videos, it would be nice just to add a commentary and tell something about it.
About the tags - steemcleaners are not flagging tag abuse, at least if it's not intentional and continuous abuse. It's a good thing you think and try using relevant tags, this is your input in making this platform better.
Thank you!