Reference: As-Sharh Al-Mumti’, Vol. 4/129-130
15. The Adhaan and the Iqaama have not been prescribed for any of the Nafl prayers, as opposed to the Fardh prayers;
16. The Fardh prayers are shortened throughout a journey, as for the Nafl prayers then they are not shortened throughout the journey;
17. It is allowed for a person to leave the Nafl prayer in cases of inability and yet the reward is written in full for the one who was consistent in performing them. As for the Fardh prayer, then it cannot be missed in any situation. However, the one who is not able to perform it perfectly due to an inability, then the full reward is written for him if he was consistent in performing it;
18. To make specific acts of remembrance (dhikr) has been prescribed after all the Faraaidh prayers. As for the Nawaafil prayers, specific acts of remembrance (dhikr) have been prescribed in some instances and not others;
19. It is permissible to pray the Nafl prayer inside the Ka’bah; however it is not permissible to pray the Fardh prayer in it. [This is one opinion of the scholars however] the correct opinion is that it is allowed to pray both prayers inside the Ka’bah without any difference;
20. To pray in congregation is obligatory for the Faraaidh prayers as opposed to the Nawaafil prayers;
21. A person is allowed to combine the Faraaidh prayers  as opposed to the Nawaafil prayers;
22. The Faraaidh prayers are greater in reward than the Nawaafil prayers;
23. The permissibility to drink a small amount in the Nafl prayer, as opposed to the Fardhprayers [where it is impermissible]; 
24. From the Nawaafil prayers are those that can be prayed as one single unit,  as opposed to the Faraaidh;
25. It has been prescribed for a person to ask Allaah or seek refuge in Him when reciting a verse of mercy or a verse of punishment. This is to be done in the Nafl prayers. As for the Fardh prayers then it is permissible however not necessarily prescribed nor desired; 
26. It is allowed for the adult above the age of puberty to be led in the Nawaafil prayers by the young child under the age of puberty. This is the case in the Nawaafil prayers and not the Faraaidh. [This is one opinion of the scholars however] the correct opinion is that this is allowed in the Faraaidh prayers as well – and there is no difference;
27. The permissibility of the one who is praying a Nafl prayer to pray behind the one who is praying a Fardh prayer and not the opposite. [This is one opinion of the scholars however] the correct opinion is that both are permissible and there is no difference;
28. From amongst the Nawaafil prayers are those that are, if missed, prayed in their original form and some of them are prayed differently – like the Witr prayer; 
As for the Faraaidh prayers, if missed, they are prayed in their original form except the Jumuah prayer; if it is missed it is prayed as the Dhuhr prayer;
29. The Faraaidh night prayers  are to be recited aloud, as for the Nawaafil prayers in the night then a person has the right to choose between reciting aloud or not;
30. The obligation to cover one’s shoulder in the Fardh prayer and not the Nafl prayer – according to one of the opinions;
31. From amongst the Nawaafil prayers are those that are exempted whilst on a journey. As for the Faraaidh prayers then nothing is exempted from them.
 In times of necessity such as the journey.
 This is the opinion of some of the scholars of the Hanbalee Madhhab, based upon the narration collected by Ibn Al-Mundhir: Abul Hakm narrates that he saw the companion Abdullah bin Zubair drink water whilst in prayer. This is because he would lengthen his voluntary Nafl prayer to such an extent that he would feel extreme thirst and therefore drink a little amount of water.
The correct opinion however – and Allaah knows best – is that there is no difference between the Fardh and Nafl prayer in this regard and therefore it is not permissible in any prayer to intentionally eat or drink whilst praying, neither a little nor large amount.
Shaykh Ibn Uthaymeen said, whilst discussing this very issue: ‘It is clear [to me] that there is no difference between the Fardh and Nafl prayer; whatever invalidates the Fardh prayer also invalidates the Nafl prayer. The principle in this regard is that they are both similar in rulings.’
Please refer to the following discussion in Arabic for more light on the issue: http://www.sahab.net/ipb/index.php?showtopic=104115
 Such as the Witr prayer.
 Shaykh Ibn Uthaymeen holds this action to be an established Sunnah of the prophet (sal Allaahu alayhi wa sallam) in the Nawaafil prayers; he also holds it to be a permissible act in the Faraaidh prayers although not established as a Sunnah. Please refer to Vol. 3, p. 288 in the same book for further information.
 If one misses the Witr prayer forgetfully and wishes to pray it the following day, he prays it as even units of prayer as opposed the original form of praying odd units.
 Referring to Maghrib, Isha [and Fajr]
الجزء الثاني – الأمورُ التي تفارقُ فيها النوافلُ الفرائضَ
المرجع: الشرح الممتع – المجلد الرابع ص: 129 – 130
15) لا يُشرع الأذانُ والإقامةُ في النَّفلِ مطلقاً، بخلافِ الفَرْضِ؛
16) الفريضةُ تُقصرُ في السَّفرِ، أما النَّافلةُ التي في السَّفر فلا تُقصر؛
17) النَّافلةُ تسقطُ عند العجز عنها، ويُكتب أجرُها لِمَن اعتادَها، والفريضةُ لا تسقطُ بحالٍ، ويُكتبُ أجرُ إكمالِها لمن عجز عنه؛ إذا كان من عادته فِعْلُه؛
18) جميعُ الفرائضِ يُشرعُ لها ذِكْرٌ بعدَها، أما النَّوافلُ فقد وَرَدَ في بعضِها، وفي بعضهِا لم يردْ؛
19) النَّافلةُ تجوزُ في جَوْفِ الكعبةِ، وأما الفريضةُ فلا. والصَّحيحُ جوازُها فلا فَرْقَ؛
20) وجوبُ صلاةِ الجماعة في الفرائض، دون النوافلِ؛
21) الفرائضُ يجوزُ فيها الجمعُ، بخلافِ النوافلِ؛
22) الفرائضُ أعظمُ أجراً مِن النوافلِ؛
23) جوازُ الشُّربِ اليسيرِ في النفلِ، دون الفرض؛ 
24) أنَّ النوافلَ منها ما يُصلَّى ركعةً واحدةً، بخلافِ الفرائضِ؛
25) يُشرعُ في صلاةِ النافلةِ السؤالُ والتعوُّذ عند تِلاوة آيةِ رحمةٍ، أو آيةِ عذابٍ، وأما الفريضةُ فإنه جائزٌ غيرُ مشروعٍ؛
26) جوازُ ائتمام البالغِ بالصَّبي في النافلةِ، دون الفريضةِ، والصَّوابُ جوازه فلا فَرْقٍ؛
27) جوازُ ائتمامِ المتنفِّلِ بالمفترضِ، دون العكس، والصَّحيحُ جوازُه فلا فَرْقَ؛
28) النَّوافلُ منها ما يُقضى على صِفته، ومنها ما يُقضى على غير صِفته كالوِتر، أما الفرائضُ فتُقضَى على صِفتها، لكن يُستثنى مِن ذلك الجُمعةُ، فإنها إذا فاتتْ تُقضى ظُهراً؛
29) صلاةُ الفَريضةِ الليلية يُجهر فيها بالقِراءة، أما النَّفلُ الذي في الليلِ فهو مخيَّرٌ بين الجهرِ وعدمِه؛
30) وجوبُ ستر العاتق في الفريضة على أحد القولين، دون النافلة؛
31) مِن النوافلِ ما تسقطُ بالسَّفَرِ، وأما الفرائضُ فلا يسقطُ منها شيءٌ.
 للتفصيل في المسألة راجع: http://www.sahab.net/ipb/index.php/showtopic=104115