Well there is a variety of gamemodes.
Some servers are CTF, some are DM (and suck), others are TDM, but most servers will be co-op games.
Of these co-op games, the most servers will usually run Evolution, wich basically has you and your buddies clearing out a whole lot of towns on the island, and i really do mean a whole lot. Next to that, there are also side-missions that can be taken on etc. It's pretty impressive, but if you join it chances are you wont find anyone unless your there from the start, or figure out how to join a squad and actually have them pick you up.
Usually, i'd say dont bother unless you are there from the start, or have some people you know, and team up with them (wich Nimsky and me used to do on these servers, just our 2-man squad taking on side-missions wich is totally fine).
If you want, you can join the co-op group we made here on the forums (found in social groups, link to steam group can also be found) or add me on xfire: sicdisaster so we can arrange a co-op session sometimes
If you take a look at the custom missions/plans page on my website, you'll find a link to the XAM 1.4 mod at my custom mission for ArmA.
What it exactly does is update a whole lot of graphic effects like smoke, fire, explosions, dirt in your face when you jump to prone from a sprint, dirt in your face after you've been to close to an explosion (comes with ringing ears and flashing of the screen), to screenflashes when bullets go by real close (fear effect, kind of resembles blinking your eye i guess).
It also changes allmost, if not all sounds for weapons. And honestly, they sound so damn great (especially m4 rifle) i dont want to go back to vanilla ArmA
It also adds a few custom islands, and desert camo-vehicles.
They added radio's to helicopters with old vietnam-era rock tunes and Wachner's classing assault song among others
Also comes with adjusted difficulty settings, you have a minor realism setting leaving ArmA pretty much as is, but on extreme simulation setting the mod takes away ALL gamey indications.
Outlined targets (when your given one by the commander for example) disappear, as well as mission markers so you'll have to read a map, if a soldier calls out a soldier at 3 o'clock the little clock in wich you can see that 3 o'clock doesnt pop up anymore and other stuff i cant think of right now. Basically, if you want to know something, you'll have to actually gather that information yourself instead of just getting it indicated and hinted at for you.
Oh, and i didnt even mention bleeding and first-aid. Those might not be the most realistic things in some people's opinions but it hardly happens anyway. Think of a shot to the arms causing you to bleed but still able to operate your weapon, you'll have to stop the bleeding by binding etc.
A patch for use if you want to play 1.09 beta is also available at that download link.
All in all, it really is an impressive mod and you wont be let down by it.
Below the mission file you can see a video showcasing the mission, using the desert camo vehicles, a custom island, different camo skins for units (far more realistic-looking) and you can hear a lot of the improved sounds