It depends on the ship. Only Oasis and (some) Quantum class come with Voom. All ships will have either a suite lounge or a concierge lounge, where select beverages are complimentary during set hours (430pm to 8pm, give or take 30 minutes). Room service will be free, and you will be able to order from the Main Dining Room menu for room service (during meal times).
A GS stateroom is an excellent size, usually with an actual tub in the bathroom. You will also have upgraded bath amenities (shampoo, lotion, soap). There will be robes for use onboard. There will likely be a suite welcome/cocktail party with ship officers in attendance; you will get an invitation in your cabin. Also, depending on the ship you may be offered a free behind the scenes entertainment tour or bridge tour.
I'm probably forgetting something, but I'm sure others will chime in!