Basically, there is no set or defined rules to identify any newly introduced thing as Halal or Haram (which is not specifically mention in the Quran and Hadith). Otherwise, anyone can become Mufti and publish his own Fatwah.
Every Mufti judges on their own study, understanding, their effect in the society, cultural behaviors, long-run effects in the society, practical effects observed on the next generations. This is why we see different Fiqqah's in Islam. Otherwise, there were only one Fiqah.
This is same like, if you pass LLB Degree and you can become a lawyer. But you are not a Judge. You spend a time in practice then if you get appointed in Session Court then you become a Judge. But still, you are not final Judge to declare decision. Public can take or challance your decision to High Court and even to Supreme Court. Every Judge has different skill & experience level and angle of observation. Same applies to Islamic Mufti.
If you see, each Mufti/Scholar may have different educated opinion and recommendations on a matter.
A fatwā is an Islamic legal pronouncement, issued by an expert in religious law (mufti) where Islamic jurisprudence (fiqh), is unclear.
We might compare a fatwā to the legal ruling of a high court or the Supreme Court, depending on the authority of the mufti behind it. However, a fatwā is not binding as is the verdict of the secular courts; while correct and applicable to all members of the Muslim faith, the fatwā is optional for the individual to respect or not.
It is not allowed for anyone to give a Shari¿ah explanation (fatwā), except one
who knows the Holy Qur’ān completely including what verses are abrogated and by which verses they were abrogated, and which verses resemble each other in the Qur’ān and whether a chapter was revealed in Makkah or Madina.
He must know the entire corpus of the Hadith of the Prophet (s), both those which are authentic and those which are false. He must know the Arabic language of the time of the Prophet (s) with its grammar and eloquence as well as know the poetry of the Arabs.
Additionally, he must know the culture of the various peoples who live in each different nation of the community.
If a person has all such attributes combined in himself, he may speak on what is permitted (ħalāl) and what is forbidden (ħarām). Otherwise he has no right to issue a fatwā.
It often happens that different Islamic clerics issue contradictory, or competing, fatwās. This divergence of opinion is not considered an issue in Islam; in fact, a well-known saying states that such differences among scholars are God’s mercy, for they allow for different conditions and temperaments among people.
Also, the luxurious cars fatwa reference is addressed to only clergy and clergy students as mentioned here: http://english.khamenei.ir/news/4633/It-is-haram-for-the-clergy-to-use-luxury-cars-Ayatollah-Khamenei
Here is another version of Fatwa and declared luxury life as Haram (which you can follow or unfollow on some bases): http://www.khilafah.com/is-it-forbidden-to-live-a-luxurious-life-in-islam/