This expansion doesn't add much to the original Quake II, but it's enough to considerably change the gameplay.
This add-on doesn't add much to the original Quake II, but the added content efficiently makes the game harder and more slow-paced. It's harder because of both the amount of enemies and the added monsters (as well modified versions from the original game), and a bit slow-paced because you can't simply enter in a room and continually kill stuff. The most noticeable addition in Ground Zero are the auto-turrets, which will make the player to move more slowly and look carefully at the walls and ceiling, since those mechanized weapons are very well placed, hard-to-spot and dangerous ones.
The auto-turrets can be really annoying sometimes, since you must break the pace and start searching for them every time. And that doesn't guarantee that you won't be backstabbed by an auto-turret, since some of them are activated only before you enter a specific area. This isn't fun at all and will only make you quick load a lot, which is particularly bad, because Quake II is a game that you'll want to freely and incessantly move in between rooms and corridors, gibbing the hordes of aliens that are creeping around the Stroggs complexes. In my opinion, doing that without any stop is much better.
The new levels are great, and consumed 10 to 12 hours of my free time. Maybe it would be less if I hadn't played on 'Hard' difficult, which I found challenging in almost all the maps. Also, the enemies have a better Artificial Intelligence, and they even fire against you if you take cover, shooting at your last known position. They also chase you, although is much like the mindless ghosts in Pac-Man, and it's really easy to surprise them while you wait in a corner with a double-barreled shotgun.
There are new weapons and items as well: the chain saw remembers a lot the old DOOM; the ETF Rifle is a remake of the nail-gun from the first Quake; the plasma beam may be compared to the original lightning-gun; the proximity mine can be used to set traps against your enemies; the tesla mine electrifies nearby monsters; and the defender sphere, which adds protection from attacks and also fires energy bolts at the targets. There're also two spheres only for deathmatch, both of them retaliates the attackers that either hurt or kill you. Another power-up exclusive for the competitive multiplayer is the doppleganger, used to create a hologram of you to cheat opponents.
I almost didn't use any of the new weapons and items, so you can bet that they aren't crucial for your success in Ground Zero expansion. Nevertheless, if they aren't that useful in single player mode, most of the new additions are quite helpful and even necessary in multiplayer. Not to mention the new deathmach maps, which Rogue Entertainment certainly did a great job.
So, it's true that the new content is a bit thin, but it's enough to change the original gameplay and to make it even better sometimes, especially about the multiplayer mode.