The importance of having good manners as a Muslim isn't something that is considered an "extra", it is an essential part of our faith. All too often we find people who pray Salah, know Quran and Hadith, fast in Ramadan etc. but when you see them they have a mean look on their face, or they don't return Salams, etc.
Whether it is Salah, Sawm, Sadaqa, or any of the other beloved deeds that Rasulullah (S) has told us to perform, they should all make us more humble and more kind, more well mannered and grateful to Allah (SWT), not more rude or arrogant

Abu Huraira reported: The Messenger of Allah, peace be upon him, said, “Verily, I have only been sent to complete righteous character.”

[Musnad Ahmad, Number 8729, Sahih]

عَنْ أَبِي هُرَيْرَةَ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِنَّمَا بُعِثْتُ لِأُتَمِّمَ صَالِحَ الْأَخْلَاقِ

8729 مسند أحمد بَاقِي مُسْنَدِ الْمُكْثِرِينَ إنما بعثت لأتمم صالح الأخلاق

المحدث السيوطي خلاصة حكم المحدث صحيح

http://www.youtube.com/watch?v=IH2_0_IWUN0