ICO is just a beginning of the road. Of course, hitting hardcap means successful ICO but the main idea and the project comes after that. So if we speak about ICO - yes successful ICO is when hitting hardcap but the project itself will be successful only when a product and main idea is delivered and presented for the general public (or investors).

But what is successful project? Usually, token prices decrease because of the community, the saddest part is that we have a lot of newbies in ICO's and they hardly understand how things work. The price fall down and everyone blames ONLY the team but there is a lot more behind it. No patience and tolerance bring more pressure on the team and sometimes (if team cares) they start to do mistakes by doing things faster for an impatient community. Saw that too many times tbh. However, I think success of a project is measured by delivering the highest quality product.