This is my opinion from what I understand and from my studying of Shias and their disputes with Sunnahs.

It is very hard for a non-Muslim to understand the real differences between Shia and Sunnah.

Some Shia groups are close to Sunnah beliefs and have a lot in common, but these are a minority and are themselves rejected by the majority of Shia groups enough to be considered traitors.

The Shia believe that the real prophet Allah wanted was Ali (prophet Mohammed's cousin) and that the angel Allah sent his message with (Gabriell) betrayed his mission and sent it to Mohammed (Not by mistake as many think).

They believe that the holy family of Ali (which includes Mohammed) are sacred, perfect and can never sin or mistake in general. They call them "Aal Al-Beit"

They especially believe that 12 members of the holy family are so sacred and holy that they posses powers resembling those of God (Allah). Many of the extreme groups believe that these twelve members (called the "12 A-Emma" meaning the 12 Imams) control the universe, decide the final day, who goes to heaven and who goes to hell. They practically make them the ultimate power of this universe!

This is the biggest contradiction Shias have with Sunnahs who believe only in the one ultimate immortal power of God (Allah) who does not share this with any other creature whatsoever.

The Shia believe that 11 holy members of the sacred family have already passed away and one is still to come and save the world.

Their dispute with the Sunnah begins as early as the prophet was wrongly appointed in their point of view.
They also consider that all Mohammed's friends and colleagues were betrayers except for a small number of individuals (this means that almost 125,000 colleagues of Mohammed are considered betrayers and non-believers who repelled from Islam except for 6 to 10 individuals in their point of view).

Even their rituals are different from Sunnah's!

They generally do not consider the holy pilgrimage to Mecca, but to the tomb of Al-Hussain (Ali's son and one of the 12 holy Imams). Some of them refuse to pray unless they have a portion of sand from Al-Hussains tomb area to land his/her head on.

They consider that the Quran we have between our hands is false and that the real Quran is the one Fatima (the daughter of Mohammed and wife to Ali) had hidden.
In their opinion, it is very different to the one the Sunnah have.

Plenty of Shias don't reveal this belief because of the following:

They have "Al-Tukia" principal which means to behave and do something while believing the opposite. They can treat you well and consider you a friend while believing you are their enemy and go on with this for a life time.
It is considered part of their religion.
Sunnah's consider this as Hippocratic.

They are famous in this to gain followers and have succeeded in many parts to expand their powers. This is nowadays a political art of the Iranian government.

Examples of expansion are: Iraq, Pakistan, Lebanon, Kuwait, Bahrain and Qatar.

Here in Qatar, I believe the government is trying to reduce their influence and authority as much as possible after having seen what they can do when they get to authority. The biggest example is Iraq.

In Iraq, the Shias have taken the ruling part and are now an ally to Iran. The friendship with the Amercians is only a "Tukia" means to get authority and kick out the Sunnah from ruling.

This may sound strange, but only time will tell what I am claiming here...

*********************************
When dealing with others: Never judge a person from his first act
When dealing with myself: You don't get a second chance to make a first impression.