Don't buy UT2k4 now, it will just be a waste of your money if you plan on getting RO: Ost. The retail product is superior to the mod in every aspect.
Red Orchestra focuses on realism. The first thing you will notice apon firing up RO is you have no crosshair, no option to turn on a crosshair, and no way to get a crosshair what so ever. In the words of our president:
"There are no crosshairs, and there is no option to turn on crosshairs. We feel in the evolution of the tactical first person shooter our game has gone far beyond the need or usefulness of crosshairs. Obviously when your aiming down the sight of the gun there is no need for a crosshair. But, even when aiming the gun from the hip, there is a "free-aim" system where the gun moves independent of the players rotation. The bullets go exactly where the barrel is pointing, not the center of the screen. So just like in real life, you estimate where you think the barrel is pointing, and use the bullet puffs hitting the ground to walk to shots onto the target.
It takes a little getting used to, but for most players its not to hard to pick up. One of the big benefits to gameplay of not having crosshairs is it encourages poeple to use thier weapons realistically. Firing on the move is much less effective, and the game plays more tactically. This in turn puts more emphasis on tactics and movement than it does on twitch reaction time."
The free aim system RO uses keeps the gun from pointing at the center of the screen. For example if you move your mouse to the left you'll notice your gun barrel travels far more to the left then your character turns to the left.
More aspects that RO focuses on are a momentum system where characters don't just jump to full speed, true bullet physics with drop and power behind the round, a hud with minimal details to keep the game fun and running (IE: no bullet count, when you reload you get a small message saying if the magazine is heavy or light depending if it is above half full or not).
Teamwork while not required is rewarded. If you run off by yourself you will soon find yourself dead by the hands of a group of enemy soldiers who easily took you out. Moving with a group and having others watch your back if your a sniper or machine gun will keep you alive much longer.
There are many different classes, some which share traits, others which stand by themselves. The commander classes (both infantry and tank) are equiped with binoculars which allow them to spot artillery targets, and later call them in which they reach a radio. The machine gunners have the machine guns obviously, and the snipers the sniper rifles. From here there are your basic rifle infantry, smg infantry, and then we have such units as the sappers who are very much like smg infantry however they carry explosive satchels. Each class offers somthing unique, and you'll want to try them all to find the ones you like best, though all are fun.
There is only one gametype. This is to capture the enemy's positions. This can range from one team attacking the entire map on several positions in succession, or several possitions at once, to both teams attacking and defending positions. The key is to have a greater presense in the area then the enemy does. Also, while not objectives several maps feature areas where sappers can create alternate paths by using their satchels. Also RO features a range of battlefields from infantry (both city and wilderness settings) to combined arms with both tanks and infantry supporting each other like the recently shown off map RO-Konigsplatz, to tank only where tanks battle each other for supremecy.
That brings up tanks. RO tanks, while not a full sim level are quite close (and we don't make anybody play the lovely job of reloader). The three main positions for most vehicles are driver, comander/gunner, and machine gunner. Having 2 to 3 of these positions filled, while not essential to running a tank, will give you a far greater advantage over a person who has just jumped in a tank by himself and is trying to man all three positions.
I hope this counts as a brief overview