ambience
-correct lighting for the room. ambient lighting is best (not harsh spot lighting).
-color coordination/themed area
-lighting (on keyboard, mouse, computer case...)
-posters, prints, printed mousepads, anything gaming themed which makes the area more comfortable for you.
ergonomics
-comfortable chair (if you do buy a racing chair get a need-for-seat version since they have a metal frame)
-correct height desk with enough room for everything you need
-correct height keyboard/mouse surface (i personally hate using those trays and like using the tabletop itself)
-a quality keyboard (i'd suggest mechanicals)
-a comfortable mouse with buttons where you want them.
-a decent sized mousepad
-having your monitors at the correct height
visual
-playing on a big screen (like a tv... for an example see my signature link) can be very immersive
-playing across multiple monitors for peripheral vision (if your hardware supports such resolutions)
audio
-having a quality audio system which can provide volume and realistic sound
-having surround sound over stereo audio for positional info
-if using headsets/headphones investing in something nice which has high quality sound (i personally would suggest an ath-m50s and a desk or clip on mic but thats my own preferences)