Quake 4 is your standard run 'n gun shooter with a few cherries on top. First off, the graphics are great, and look perfect. Sound quality is good as well, though nothing worth praising. Pretty standard sounding.
Gameplay is sharp and smooth, which you need in this game that needs good attention. The game is dark. Few lights around. You do get flashlights on your guns, but only a couple, and later in game you'll want to use the guns that have the hit and not want to have to sacrifice that just to use some light.
Unlike it's sister Doom 3, you get marines on your side. No more soloing it out there for you. Most marines are just there for extra gun play, but some will replenish your health and others your armor, which is a huge plus.
You have plenty of escort missions, but most are escorting marines who could probably make it back to the destination themselves.
Boss battles can end pretty quickly, actually, as long as you use the righ tools, but they're still fun to go through.