For those few obscure off the grid people, Ultron is the main antagonist of Avengers 2. He is an incredibly dominant robot. His armor is made of adamantium, and is nearly unstoppable. Inevitably The team will defeat him, but for a time before he comes back in some way. The problem is that he is being introduced too soon. Right now they are theoretically just getting rolling with the MCU, and have a lot more pieces to introduce. If so, introduce a slew of villains too just to ratchet up plot potential. By doing it like this there isn't quite enough build up to it. More or less the ultimate enemy, he could be almost a 4 part movie series of constant battling. By doing it like this you are setting him up to be a main antagonist for quite a long time, while introducing him later brings more of a climax.