is this all similar to how many muslims consider shias to be nonmuslims? I actually hear it all the time from my own friends and especially from people on online forums.
The scholars have different verdicts, depending upon the beliefs held by a particular person or a sub-group. To give you an example, the mainstream sunni sect considers Abubakr(RA) the best person after the family of the prophets(peace on them), still it doesn't contribute towards the essentials of faith, likewise shia minority views Ali (RA) best amongst rightly guided caliphs, still it does not figure out amongst the pillars of faith( unless one consiers it to be). I will not indulge in sectarian discussion here, cz it leads us to nowhere, besides being the wastage of time, however the two sects have major differences, upon which the consensus can't be achieved till the day of judgement, when Allah Almighty will decide the fate of the communities in the presence of their respective prophets( peace on them).
Apart from the minor irritants, major differences, which decide the parting of ways between the two sects are :-
1. The concept of Immamate held by Shia (The sunnis don't accept, cz it strikes at the very root of prophethood and takes some individuals equal to the prophets and even higher to them.
2. Mutah (temp marriage). The sunnis don't accept and hold the opinion that it was banned by the prophet(pbuh) till the judgement by the orders of Allah Almighty.
3. Compilation of Holy Quran( the two sect differ).
4. Shia consider that many companions of the prophet(pbuh) turned their faces after prophet's departure, whereas sunnis deny these allegations.
5. The cursing of sahaba by shia can't be tolerated, even by the moderate sunnis.
Now if someone declares the other person kaafir, he has to be sure of other's beliefs. The learned scholars state, a person having belief in the concept of immamate and doubting about the compilation of holy quran and its alleged tehreef can't be considered as Muslim cz these beliefs take him out of the fold of Islam. So we should be careful and must avoid generalised statements.