For me what defines a good actor from a average actor are:
1. Screen presence; do they have that charisma about them that makes me get attached(morgan freeman,jack nickleson(sp), Tom Hanks,al pacino for example). Do they have that presence to make the movie they are in better..
2. Roll Selection: Some actors have a knack for picking the right roles for themselves and their acting styles,and some do'nt(cuba gooding jr.,sad this guy has talent,but /shrug)
3. Versitility: Are they sticking to the same type of roles? I like actors who do different roles and do them well( Tom Hanks,Bale,Jack nickleson,Will Smith). If they can do a range of movies(comedies,drama,action,suspence),and not have that same mono-tone delivery in each.
4. Progression: Has the actor progressed in their careers(see all of the above),made the right choices,progressed their skills(clive owens,bale,will smith are just a small examples),crafted their abilities.
Thats just my view on what seperates a good actor from a subpar actor..