Graffiti - what is right and what is wrong?

Some graffiti look good and some don't, some are viewed upon with approval and some not... What is acceptable and what is not? What is funny and what is not? What is art and what an offense? Michael's photo here says it clearly... but are things really as simple as that?
I gathered below some interesting comments by my class.

• Added value, appropriate place.
  Nice graffiti look nice or say something nice or something somehow of value most of us seemed to agree. Scribbles which are only there to deface the buildings or hurt somebody's feelings are vandalism.
  
• Respect other people's property.
  Edi and many others pointed out the importance of being respective of other people's property. 'It's something that has to do with age. Young people don't work and don't earn money and don't understand so well the concept of leaving things which are not theirs alone.' (I haven't thought of this before but it's true, isn't it? Most of the graffiti we see on the walls of our town looks seems to have been produced by teens.) 'Graffiti art is cool We should have designated places in towns and cities where people are allowed to do it, it shouldn't be done just anywhere.'
  
• Artistic value is not a black and white concept. 'People tend to generalize things: the beautiful is art and the ugly is vandalism or obscene. But in real life things are not that simple. A beautiful graffiti on a freshly painted wall is art for most viewers, but not for the owner. A "nonsense" sculpture in our town with numerous heads sticking out of one body - is art - even though most people don't understand it and consider it an eyesore - but it's been commissioned from a professional artist so it's officially art' (Edi)
  
• Urban dialogue. 'there are many graffiti makers who simply rebel against the system or joke around - their graffiti are urban responses reflecting reality of life in the area' (Tomaž)
  I kind of agree here, because I find it interesting to observe the walls in our town and study what they say about us.
  
  For example. Our town is a mix of all sorts of nationalities from ex Yugoslavia - in addition to Slovenes there are also many people with roots from other former Republics. Awhile ago I was on my way to Qlandia, our local mall, and I noticed a disturbing and pathetic Slovene nationalistic graffiti saying 'srbe na vrbe' (suggesting the Serbs should be hung on willow trees - it rhymes in Slovene). A humorous Serbian rhyme was added next to it in response: 'nema vrba koliko ima srba' (meaning there aren't as many willows here as there are Serbian people - not too far fetched considering how small the territory of our country is ;-)). I hate the nationalistic stuff on our walls, but this one brought a smile on my face. It is an example of this sort of urban dialogues on the walls...
  
  A much nicer example of this sort recently added to the wallstalking site is this:
  
  The authorities in our town (Nova Gorica) have put these signs boasting that our town is a university town along all major roads leading to Nova Gorica. The problem is they seemed to have forgotten about their old promise to also provide appropriate location for building the university campus - so an action called 'University Town' ' was organized in which activists furnished all these signs with the question ' Where is this university?
  

Any other thoughts on this?