People who do not accept the last prophet and/or the Qur'an are disbelievers, regardless of whether they believe in God or not!

Verse 68 of Chapter 29:

And who is more unjust than one who fabricates lies against Allah, or denies the truth* when it comes to him? Is there not a place in hell for disbelievers?  

(* The Holy Prophet and / or the Holy Qur’an).

Who are the people for whom no weight be assigned on the Day of Judgement?

Verses 103-106 of Chapter 18:

Say: "Shall We tell you the greatest losers in respect of (their) deeds''. "Those whose efforts have been wasted in this life while they thought that they were acquiring good by their deeds.'' "They are those who deny the Ayat of their Lord and the meeting with Him. So their works are in vain, and on the Day of Resurrection, We shall assign no weight for them.'' "That shall be their recompense, Hell; because they disbelieved and took My Ayat and My Messengers for jest.''

Does Worshiping God mean to follow all the instructions provided by Him?

Verses 23-39 of Chapter 17:

And your Lord has ordained that you do not worship anyone except Him, and treat your parents with kindness; if either of them or both reach old age in your presence, do not say "Uff"* to them and do not rebuff them, and speak to them with the utmost respect. And lower your wing humbly for them, with mercy, and pray, "My Lord! Have mercy on them both, the way they nursed me when I was young." Your Lord is Well Aware of what is in your hearts; if you are worthy, then indeed He is Oft Forgiving for those who repent. And give the relatives their rights, and to the needy, and to the traveller; and do not waste needlessly. Indeed those who needlessly waste are brothers of the devils; and the devil is very ungrateful to his Lord. And if you turn away from these, expecting the mercy of your Lord, for which you hope, then speak to them an easy word. And do not keep your hand tied to your neck nor open it completely, lest you remain seated - reproached, weary. Indeed your Lord eases the livelihood and restricts it, for whomever He wills; He Well Knows, Beholds His bondmen. And do not kill your children, fearing poverty; We shall provide sustenance to them as well as to you; indeed killing them is a great mistake. And do not approach adultery - it is indeed a shameful deed; and a very evil way. And do not wrongfully kill any living being which Allah has forbidden; and for whoever is slain wrongfully, We have given the authority to his heir, so he should not cross limits in slaying; he will surely be helped. And do not approach the wealth of the orphan except in the best possible way, till he reaches adulthood; and fulfil the promise; indeed the promise will be asked about. And correctly measure when you measure, and weigh correctly with the scales; this is better, and has a better outcome. And do not pursue the matter you do not have the knowledge of; indeed the ear, and the eye, and the heart - each of these will be questioned. And do not walk haughtily on the earth; you can never split the earth, nor be as high as the hills. Of all this related before, the evil among it is disliked by your Lord. This is part of the divine revelations of wisdom that your Lord has sent down to you (O dear Prophet Mohammed - peace and blessings be upon him); and O listener, do not set up another God with Allah, for you will then be thrown into hell - rebuked, rebuffed.

 (* Any expression of disgust.)