The Sunni school of thought’s starting point for understanding the concept of Imamah is to give the title of Imam or Khalifah (Caliph, Viceroy) to the elected political leader of the Islamic state, whose responsibility lies in managing various aspects of the affairs of the Muslim populace. They arrived at this conclusion after (claiming) to have referred to the Quran and Hadith/Sunnah (Prophetic narrations), without having found a clear and explicit statement defining the role of the successor of the Holy Prophet (s) (we will not judge or challenge the truth of their claim here), the method of his election/appointment, and his characteristics.
Therefore, the logical step (in their view) was to refer to actual events in the post-Prophetic era on the method of electing a leader (after arriving to the conclusion that a political vacuum is detrimental to the unity and strength of the young Islamic state). Hence, Shura council was adopted as one method, then, the first incumbent (who was elected), nominated and appointed his own successor. In the third instance, the second incumbent (who was nominated), appointed a committee of six men and charged them with the duty of selecting one out of themselves as the future leader of the Muslim community.
Secondly, since political leadersip is a ‘matter of the people’, it becomes logical that the people elect their leader and manage its own affairs (it’s human nature to reject the idea of a leader being appointed without them having a say in the matter). They usually point to this Quranic verse as evidence to their logic: ‘… and their rule is to take counsel among themselves, … ‘ (al-Shura:38). This in turn leads to the temporary nature of such an Imamah, since the arrival of the Imam to power relies on his election by the people (their pledge of allegiance to him, in the old terminology), if they chose to do so.
Thirdly, when the scholars of that school came to establish the necessary characteristics of the Imam, they did not find any need for infallibility, and settled for the virtue of justice in the fiqhi (juristic) sense. And with respect to his knowledge, it would suffice for him to possess knowledge that will allow him to perform his political duties.
So we end up with the following aspects of the Sunni theory of Imamah:
1) it is a worldly matter of political leadership
2) it achieved through popular election or through a council of a few men
3) it is not continuous and is only established if the populace chooses to do so
4) it only requires the individual to possess the commonly understood virtue of justice, and not infallibility.
The consequences of this understanding of Imamah
We find that the theological method in the Shia school has not, in the most case, attempted to resolve the point of contention with the Sunni school on the concept of Imamah. Instead, it allowed itself to get dragged into a never-ending argument and counter-argument dispute, with the Shi’a scholars doing most of the counter-arguing.
Some see this as a normal debate between schools of thought, each with their own opinion on a fundamental religious matter. However, it soon dawned on them that the responsibilities of the Imam as a political leader, are not in harmony with the conditions and characteristics assigned to him by the Shi’a school, considering these conditions to be far wider in scope than required by a political leadership.
This has led to thinkers and some scholars to reject the theory of selection in Imamah. Others started to limit infallibility to a limited sphere, and unnecessary beyond it. Another group rejected infallibility in its entirety, arguing that those who advocate such a theory abandoned it during the greater occultation, allowing fallible leaders to take charge as Imams of the Muslim nation. They consider justice to be a sufficient personal quality for the Imam to do his job as required by Islamic law (some have dropped this requirement as well..).
Others consider this dispute of leadership after the death of the Prophet to be a pointless historic dispute. And finally, we need to address those who question the value of a hidden Imam, who is not able to guide and lead his followers.
The reader needs to bear in mind at all times when dealing with the above criticisms, that they come as a direct consequence of the Sunni understanding of Imamah, which is limited to political leadership.
An alternative approach
When one studies the Quran and the undisputed narrations of the Holy Prophet (s) and his Pure Progeny (as), it becomes evident that the concept of Imamah, as understood by the Shi’a school, differs fundamentally and essentially to the Sunni understanding. The school of Ahlul Bayt (as) believes that the Quranic understanding of this concept gives the Imam roles that are far more important and requiring more stringent conditions, taking it out of the fold of branches of religion (politics, the social sciences and management of the affairs of the people, etc), and into the fundamental doctrines of faith. The Shi’a school considers Imamah to have two dimensions: the first being the juristic role (tashree’i), which necessitates infallibility, and the second being the existential (takweeni) role, or ‘wilayah’ (i.e. authority over the world of existence), which necessitates special knowledge not acquired through conventional means. And this is what martyr Mutahhari calls ‘the height of Shi’a understanding’ of the concept of Imamah. 
 Imamah, by Martyr Murtadha Mutahhari, pg 44-50.