You're right. You can't compare Tehran to Istanbul: Istanbul is by far a more beautiful city and has more character and charm. Thats mainly because of the geographical location (near the Bospurus, etc) and its history. The climate is more pleasant and the air is much more cleaner. The traffic is also less dense and the architecture is nicer. The city is overall more suitable for tourist:I thought Esfahan was the cleanest and tidyest city I had seen in Iran. It really looked good. It was much better than Tehran, Mashhad or Bandar Abbas. Im not talking about just the old buildings. It has definately got many historical attractions, but the city as a whole is better looking and more pleasant for tourists (at least when the river has water in it).
For Tehran, transport and the general environment including traffic congestion and air quality needs to be sorted out before you can even think about it being a major tourist destination. Yes it has culture but so does everywhere. It has the old American Embassy but I dont think that is even open to the public. It could be an oportunity if it was used as a museum of espionage or something. All there is at the moment is the culture as you say, the palaces and the telecabin. Compare that to Istanbul.
This is what you call a real beautiful (tourist) city. And don't get my wrong: I just want Tehran to look better.