REST URL naming convention /items/{id} vs /items?id={id}

I understand that in MVC pattern and in REST services it is common to use URIs like /items/{id} but what is bad thing about using query parameters in the URI?

GET /items/{id} vs GET /items?id={id}

Further, lets say an entity has 'referenceId' field that points to some related (say parent) entity, and I need to create REST service to get all items for parent entity, which way is better:

GET(POST) /items/parent/{parentId} 

or

GET(POST) /items?parent={parentId}

Will be grateful for insights that would help to resolve my subjective issues on constructing URLs for REST services.