Punishment for Not Wearing Hijab
Question

If a girl doesn't wear hijab, does that mean she will go to Hell?  But what if she prays Salat, recites Quran regularly, acts decently, doesn't look at boys, doesn't gossip/backbite, etc, will not wearing hijab take her to hell despite all her good attributes?
Praise be to Allah.Importance of following the commands of Allah and His Messenger
First of all it is essential to know that Muslim men and Muslim women are obliged to follow the commands of Allah and His Messenger, no matter how difficult that may be for people and without feeling shy of other people.
The believer who is sincere in his faith is the one who is sincere in carrying out his duty towards his Lord and obeying His commands and avoiding that which He has forbidden. No believer, man or woman, has the right to hesitate or delay with regard to commands; rather he or she should hear and obey immediately, in accordance with words of Allah (interpretation of the meaning):
“It is not for a believer, man or woman, when Allah and His Messenger have decreed a matter that they should have any option in their decision.” [al-Ahzab 33:36]  
This is the case with the believers who are praised by their Lord in the verse (interpretation of the meaning):
“The only saying of the faithful believers, when they are called to Allah (His Words, the Quran) and His Messenger, to judge between them, is that they say: “We hear and we obey.” And such are the successful (who will live forever in Paradise).
And whosoever obeys Allah and His Messenger , fears Allah, and keeps his duty (to Him), such are the successful.” [al-Nur 24:51-52]  
Look at the greatness and might of the One Whom you are disobeying
Moreover, the Muslim should not look at how small or great the sin is, rather he should look at the greatness and might of the One Whom he is disobeying, for He is the Most Great, the Most High; He is Mighty in strength and Severe in punishment, and His punishment is severe and painful and humiliating. If He exacts vengeance on those who disobey Him, then utter destruction will be their fate. Allah says (interpretation of the meaning):
“Such is the Seizure of your Lord when He seizes the (population of) towns while they are doing wrong. Verily, His Seizure is painful (and) severe.
Indeed in that (there) is a sure lesson for those who fear the torment of the Hereafter. That is a Day whereon mankind will be gathered together, and that is a Day when all (the dwellers of the heavens and the earth) will be present.” [Hud 11:102-103]
 A sin may appear insignificant in the eyes of a person when before Allah it is serious, as Allah says (interpretation of the meaning):
“…you counted it a little thing, while with Allah it was very great.” [al-Nur 24:15]  
This matter is as some of the scholars said: 
“Do not look at the smallness of the sin, rather look at the One Whom you are disobeying.” 
We have to obey Allah and do as He commands. We must remember that He is watching in secret and in public, and avoid that which He has forbidden.  
With regard to belief, if a Muslim who prays commits some sins or bad deeds, he is still a Muslim so long as he does not commit any action which would put him beyond the pale of Islam or do any of the things which nullify Islam. This sinful Muslim will be subject to the will of Allah in the Hereafter; if He wills He will punish him, and if He wills He will forgive him. If he enters Hell in the Hereafter, he will not abide there forever. But no person can be certain as to his fate, as to whether the punishment will befall him or not, because this matter is known only to Allah.  
Categories of sins
Sins are divided into two categories, minor and major. 

Minor sins may be expiated for by prayer, fasting and righteous deeds. 
Major sins (which are those concerning which a specific warning has been narrated, or for which there is a specific hadd punishment in this world or torment in the Hereafter) cannot be expiated for by righteous deeds. Rather the one who commits such sins has to repent sincerely from them. Whoever repents, Allah will accept his repentance. There are many kinds of major sins such as lying, zina, riba (usury/interest), stealing, not wearing hijab at all, and so on.  

Punishment for not wearing hijab
Based on the above, we cannot say for certain whether a woman who does not wear hijab will enter Hell, but she deserves the punishment of Allah because she has disobeyed His command to her. 
With regard to her specific fate, Allah knows best what it will be. We cannot speak about things of which we have no knowledge, because Allah says (interpretation of the meaning):
“And follow not (O man, i.e., say not, or do not, or witness not) that of which you have no knowledge. Verily, the hearing, and the sight, and the heart of each of those ones will be questioned (by Allah).” [Al-Isra 17:36]
It is sufficient deterrent for the Muslim whose heart is aware to know that if he does a certain action he will be exposed to the punishment of his Lord, because His punishment is severe and painful, and His Fire is hot indeed.  
“The Fire of Allah, kindled which leaps up over the hearts.” [Al-Humazah 104:6-7]  
On the other hand, for the woman who obeys her Lord’s commands – including observing complete hijab – we hope that she will enter Paradise and attain the victory of salvation from the Fire and its torments. 
Useful tips 
It is strange indeed that a woman whose character is good, and who prays and fasts and does not look at boys, and avoids gossip and backbiting, does not wear hijab . If a person really achieves these righteous deeds, this is a strong indication that she loves goodness and hates evil. Let us not forget that prayer prevents immorality and evil, and that good deeds bring more of the same. Whoever fears Allah concerning his nafs, Allah will support him and help him against his nafs. 
It seems that there is much goodness in this Muslim woman, and she is close to the path of righteousness. So she should strive to wear hijab as her Lord has commanded her. She should ignore the specious arguments and resist the pressures of her family. 
She should not listen to the words of those who criticize her, and she should ignore the specious arguments of those sinful women who want to make a display of themselves according to fashion, and she should resist the desires of her own self which may tempt her to show off her beauty and feel proud of it. 
She should adhere to that which will afford her protection and modesty, and rise above being a mere commodity to be enjoyed by every evil person who comes and goes. She should refuse to be a source of temptation to the slaves of Allah. We appeal to her faith and her love for Allah and His Messenger, and we urge her  to  observe the hijab enjoined by Allah and to obey the commands of Allah (interpretation if the meanings):  
“and not to show off their adornment.” [al-Nur 24:31]
“and do not display yourselves like that of the times of ignorance, and perform As Salah (Iqamat as Salah), and give Zakah and obey Allah and His Messenger.” [al-Ahzab 33:33]  
And Allah knows best.