I'll give an example of what I expect from each member of the team
Props:
Strong, heavy, good tackler
Loose: Quicker than the tight, takes hits well
Tight: Stronger than the loose, big hitter
Hooker:
Quicker than the Props, solid defense, can strip the ball in a maul
Locks:
Tall timber, can call and take well in the lineout, really strong pusher, second/third to ruck/maul, good tackler
(I think it really helps to have a lock who is lineout specialised and a lock who is ruck/maul specialised.)
Blindside:
Second-fastest forward, first to the ruck, gets you 5m when he runs the ball, third option at lineout
Openside:
Fastest forward, pilfer-pilfer-pilfer, clever at the back of the maul
8:
EITHER
Strong in the rucks/mauls, runs well when he breaks the line, a second 6
OR
Big hitter, doesn't go down easily, players are scared to tackle him
Scrum Half:
Gets to the ruck/maul quickly, can pass quickly and accurately, good line breaker, good box kick
Fly Half:
The most important man on the team, makes space for the guys outside him, clever kicker, directs traffic and sets up tries
Wings:
Really fast, if they can't bring a man down they can at least push him out, knows when to run and when to kick
Centres:
Inside: A jack-of-all-trades, can kick, pass or run effectively, second playmaker
Outside: Takes the ball to the line, knows when to pass and when to try and break, strongest back
Fullback:
The best tackler on the field, big clearance kick, knows when to run and when to kick