1- Ar-Riyya` is minor shirk - that does not remove one from Islaam - but it is shirk nonetheless, and therefore worse than bid'ah that does not reach the level of kufr, and also worse than major sins such as usury, sodomy, fornication, drinking alcohol, gambling... It can be major shirk, as Shaikh Muhammad ibn 'Abdul-Wahhab al-Wasaabee (hafidhahullaah) explained: Ar-riyaa` (showing off) can be from ash-Shirk al-Akbar if a person performs an action only for the people. And it can be from ash-Shirk al-Asghar if a person starts with the right intention, but then ar-riyaa` overcomes him.
2- It nullifies the action. He will not receive a reward on the Day of Judgement for it.
3- Humiliation. Allaah will command him to go to those he showed off for in this world, and see if they have a reward for him. And as the scholars explained, Allaah will also expose this person in this world.
4- It may be a cause of entering the hellfire - aoodhubillaah! - even if it is not forever.
5- It can lead to hypocrisy - aoodhubillaah! - since the hypocrites do deeds to be seen by men, while they disbelieve. And the hypocrites, of course, will be in the lowest level of the hellfire.
And refuge in Allaah is sought from falling into ar-Riyya`