Fataawa (Ruling) of Shaykh Ibn Baaz (vol. 2, p. 396)
“It is not permissible to pray behind any of the mushrikeen, including those who seek the help of anyone other than Allaah and seek support from him, because seeking help through anyone other then Allaah, such as the dead, idols, the jinn, etc. is shirk, the association of others with Allaah, glorified be He.”
And he said: one should not pray behind any imaam who is known to exaggerate concerning the Ahl al-Bayt (members of the Prophet’s household). If no such thing is known about him or any other Muslim, then it is OK to pray behind them.
Islam Q&A (islamqa . com)
Sheikh Muhammed Salih Al-Munajjid
Edited for Elaboration: The above is a fatwa, or a ruling, given by a scholar of Islam. Some of the Shia do things that are not permissible, things that are acs of kufr (disbelief) such as
1) Calling upon Ali, Hussein and Fatima along with Allah
2) Believing that the Angel Gabriel made a mistake when he brought down the revelation to Muhammad (Peace be upon him) - [they believe the revelation should have been given to Ali]
3) They curse the companions of the Prophet
These are only a few things that some of the Shia are known to do.
So in the above fatwa, the scholar of Islam is saying that "one should not pray behind any imaam who is known to exaggerate concerning the Ahl al-Bayt (members of the Prophet’s household). If no such thing is known about him or any other Muslim, then it is OK to pray behind them."
Many shia, if not all, exaggerate concerning the family of the Prophet to the extent that their entire religion is based on praising members of his family and cursing his companions. In addition to that, and more importantly, they believe many things that are contradictory to what Allah revealed, such as (as mentioned previously), the belief that the Angel Gabriel made a mistake and brought the revelation to the wrong person.
Tawheed has several elements, all pointing back to Allah as the One Supreme and Sovereign God of all. When you begin calling upon the dead family members of the Prophet, asking them for aid and help, you take yourself away from Tawheed and out of Islam. You set up rivals with Allah. When you curse those whom Allah has blessed, you commit kufr (disbelief). When you say that Gabriel "made a mistake" - you certainly become a disbeliever, as to Allah belongs all perfections and His angels are obedient and do not make mistakes.
Note that in the end, the scholar said "If no such thing is known about him or any other Muslim, then it is OK to pray behind them"
I hope this helps you further understand, if even a little.