The game was the first PS2 game to integrate voice commands with the headset. In single player, you'd be able to issue commands to your squad simply by speaking into the mic rather than menu driven commands. So things like "Disarm bomb" "flank left" were usually handled with the mic. The obvious multiplayer mic integration was squad talk. Voice chat was segregated between teammates who were dead and alive.
While not the first online PS2 game (Tony Hawk 3 holds that honor if I'm not mistaken), SOCOM was used to really push Sony's official PS2 network adapter. The adapter also came bundled with Twisted Metal Online, but the few times I went online with that game there weren't many people playing
SOCOM1 supported 8v8 online play, but games could be forced with fewer players if the room decided. I can't remember the number of maps, but there was a decent variety to keep you entertained.
I thought it introduced a great camo system...people hiding in grass and trees really meshed into their settings and it added an amazing element to the game. I don't think a multiplayer game I've personally played has pulled it off better than SOCOM1 did.
By the time SOCOM2 came out, my friends and my interest started to wane on the series. We played A LOT of the first one...at one point a buddy and myself were ranked 13th and 24th overall respectively, and the amount of time you needed to dedicate to playing to maintain a high rank and continue to climb was much more than I could commit.
I never played much of SOCOM2's single player, and didn't play enough of the multi to really get into it. There were some new levels that were really fun like Foxhunt, and there were some new additions like adding remote devices. A new feature allowed you to play practice multiplayer games with LAN'd up PS2's which we did a number of times.
That's the extent of the input I can give on the series. I gush about the first one because it was an amazing online console game experience when taking consoles online was still treading water (The DC and SegaNet I think helped bolster the support to allow Sony and MS to take it to the next level). The online community was great until cheaters began ruining the fun of the game, and voice chat was not flooded with a 12-year-old's racist rants. Good times were had by many
