1. Both Shia and Sunni scholars emphasised on the authenticity of all the narrators of every Hadeeth, but they differ when they come across some narrators who are controversial e.g. Abu Huraira, Samarah Ibn Jundob, etc who are considered as accepted by many Sunnis while not accepted by Shia scholars because of their behaviour against Ahlul Bayt(AS).
2. Shia scholars follow the most strict procedure of authentication of every narrator whether Shia or Sunni.
3. Shia scholars take Ahlul Bayt as the main source of the authentic Hadeeths from the Prophet (SAWA) not only because they are the most pious and most knowledgeable about the Real Sunnah of the Prophet (SAWA) as they lived with him, but also because of the order of the Prophet (SAWA) to all Muslims to keep following Quran and Ahlul Bayt (SAWA) (Saheeh Muslim, Hadeeth number 4425, and Tirmithi; Hadeeth number 3718 and 3720, and al-Mustadrak by Al-Haakim al-Nisabori; Hadeeth number 4576 and hundreds of other Sunni and Shia books.
4. Shia scholars have written the Hadeeths right from the time of Prophet Muhammad (SAWA) and through the times of all the Infallible Imams (AS), while Sunni scholars were forbidden by their rulers from writing any Prophetic Hadeeth from the time of Saqeefa till the time of Umar ibn Abdul Azeez who was first Sunni ruler who lifted tha ban on writing Hadeeths, which means that Sunni scholars did not write Hadeeths for around hundred years. That has definitely affected the authenticity and the accuracy of their narrations.
5. Sunni rulers used to interfere in religion to suit their political aims. Mu'awiyah for example used to pay huge amounts of money to some narrators to fabricate narrations in his favour and against Ahlul Bayt (AS). He used to kill or torture scholars who narrate any Hadeeth in favour of Imam Ali (AS). This corruption went on during tyrant governments of Bani Umayyad and Bani Abbas. Shia Hadeeths since beginning till now are pure from any political corruption.
6. Shia scholars following the teachings of Ahlul Bayt(AS) do not accept any narration which contradicts Quran, while you find many Sunni scholars narrate narration which are clearly against Quranic verses and claim than authentic narration is accepted even if it contradicts a Quranic verse. For details you may see ( A review of the two Saheeh ( Bukhari and Muslim) by Shaikh Sadiq Najmi and ( (Quick look at the two Saheehs, by Shaikh Aasif Mohsini and many other books in this subject).
7. Many Sunni books narrated from open enemies of Ahlul Bayt and considered their narrations as authentic. Bukhari narrated from Imran Ibn Hattaan who used to praise Ibn Muljam, the murderer of Imam Ali (AS), and also from Uthman ibn Hareez who used to openly curse Imam Ali every day. This shows that some non trusted narrators have penetrated in some Sunni books and taken as trusted narrators. This has added to the difference between Muslims.