Disclaimer: I'm an ER nurse with very little experience in the ICU.
In the ER, when we have very critical patients our job is to save that patient from imminent death and then stabilize them enough to go to the ICU (or another unit). We rarely have a patient who stays with us more than two or three hours and at that point they're usually very very sick. I live in California so we have patient ratio laws. My ratio in the ER is 4:1 and in the ICU it's 1:1 or 2:1, so as the name implies, they get much more intensive care. There are certain instances in the ER when I need to be 1:1 with a patient, such as when I'm giving tPA, prepping a patient for surgery, or have a very sick patient who'll end up going to the ICU.
The thing I enjoy about the ER is never knowing what's coming through the door. I get to be there with the patient from the beginning and I sometimes get to play a role in saving their lives when death is almost certainly imminent without immediate treatment. In the ICU (and other medical floors) the nurses get to review our charting and have an idea of what's going on with the patient and where treatment will likely go.
There is a much more team oriented environment in the ER, not just between the nurses and techs but with the MDs as well. The doctors expect a certain level of critical thinking from you and most will expect you to anticipate orders and begin before they see a patient (i.e., draw labs, place orders for labs, start fluids, etc). I'm not sure how it works in ICU, but I imagine there are protocols and what not.
Both are critical care departments. You can think of the ER as the place patients go to get stabilized and then go where they need to go (either home, medical floor, or ICU) and the ICU as a place they go to get treated and made well. What I like about ICU nurses is that they make me think. When I first started and had to give report to ICU nurses they asked (and still do) questions that make me really know my patients. Both departments have many nurse driven protocols so you don't always need to call a doctor for an order. They both have a higher level of autonomy that isn't found in a med-surge unit.
In terms of pay, I don't think hospitals base pay on what department you work in.