What is the full list of effects that fields have?

Regardless of the field situation: You can collect XM. Resonators decay at 15%/day after 3 days. Hacks work the same way (item level is -1 or +2 from the min of your level and the portal level) and the item drop chances are the same.

You can't link from a portal inside an existing field to any portals. You can go to an anchor of a field, and if the anchor is not inside another field, you can link to a portal inside the field created by the anchor (given the usual non-field conditions such as fully deployed, right keys, not crossing links, etc.)

Example: One can make this ghastly blue field from the outside in or the inside out. The blue portals extending to the right of the Strasburg city label are inside the largest field. As you move right, each portal is inside a field anchored by the portal to its left.

Say you start by making the outer triangle, anchored by the leftmost of the inner portals (call it A) and the anchors of that field to the north (N) and south (S).

Look at the next portal to the right of A (call it B). You cannot go to B and link to N or S to make the next largest field - the error will read "Portal is within existing field". You must link to B from N and S.

(One would not really drive back and forth between the north and south portals to make one field at a time, so suspend disbelief for a minute. The efficient way to do this, given all the keys to A, B, C... is to go to N, link to A, B, C... then go to S and do the same.)

You can't make any new links or fields from inside an existing field, that's the only visible effect.

Some rumors say that sleeping under an enemy field is almost impossible and can lead people to drive many kilometers to destroy anchors :)