On the PC you can... just now I played on a server without the Chopper... you still get it when you kill 6(?) guys in a row, but once you call it.. you can see it it flying into the map and directly out of it again not killing anyone. And 2 days before I played on a map with chopper enabled... but even on hardcore he didn't make more than 1 or 2 kills because he goes down directly when 1 or 2 guys decide to shoot at him with their MG or even MP
(On both servers no mod was running, I played the normal Call of Duty 4 Modern Warfare, at first I thought they forgot to include heli-waypoints but then a console message appeared "HELICOPTERS ARE DISABLED" which was repeated every 3 or 4 minutes)
You can change a lot.
you can en- or disable grenades.. allow certain power ups or disable them.
You can enable a strategy time whenever a new round starts (so you can change your class for example.. INGAME!!) however some of those settings requires to download a mod... and some do not (like the one with the chopper)
The mod however is not very big... maybe 2-3 MBs
one mod for example is called "Mod -ern- Warfare" whenever you join a server with that mod enabled it will change to it automatically and if you join a normal server after that, it changes to normal mode automatically.
Its not like you ruin the game if you download mods ^^
Well... all that was possible in Call of Duty 2.
You can limit certain weapons (for example only 1 sniper in each team) limit grenades or disable them fully. allow smoke or not... its up to the server. I don't know if they are ranked though but I was able to gain EXP on every server except those who require a mod (which I don't join because they have league settings sometimes.)