This is not meant to be a balance discussion.
I am not going to claim to be any great shakes at sword and board, though I will have a good match every once in a while.
pitfalls and tendencies
FF prone - dealing and receiving. Mostly due to the shorter nature of the weapons.
Tunnel vision - the need to maintain a shield in front of the current opponent can easily lead to tunnel vision. Remapping of the look around key may help.
RMBitis - finger locked on RMB can lead to stubborn releases.
Getting swarmed. I find it easier to deal with multiple opponents with anything BUT sword and board. I am not sure if turn speed is slowed when holding RMB, it used to be.
Crush Through - sure you CAN deal with it when one versus one, how often does that hapen? Possibly the silliest mechanic ever.
Back pedallers. You may have to content yourself with getting one strike in and maintaining position, or risk getting swarmed due to over pursuit.
'Spammers' the guys who don't give you a chance to strike back. Really need to discover their timing, unfortunately, this can lead to a long delay.
Delay - this can be good or dreadful. It can take a long time at times to kill someone, which ties you up.
ROLES
Roadblock. Turtle in heavy armour, strike only when necessary, but be a bulwark for your team mates to work around. Can infuriate some people.
Guard. Guard an archer or group of ranged, intercept incoming; essentially watching their backs.
daggger and buckler assasin, with light armor.
Pushing the front.
Advancing on archers. This is kind of key; hiding behind the two handers is common, but improper. Of course, those speedy bastards do not wait for you.
Ability to switch roles / mode