I've found it interesting that the whole tone and feel of the show in the early seasons (esp 1-3) is very different from 4-9ish and now that I'm on season 10, it seems to have changed again. In fact the change in feel of the show seems to coincide almost exactly with each time they re-fit/re-design the ER (moving the reception desk, closed to open-pan, moving the doctor's lounge, waiting area, triage cubicles etc).
If I'm honest, my interest is starting to wane now - almost is exactly at the point I stopped watching first time round. I really want to see this re-watch through though and am scared to take a break in case the set disappears from 4.