Picking your team can be a lot easier if you know what to look out for.
That said, not all positions in the game are created equal, some have more importance than others, so I will attend to those first.
NB: Key Skills are in order of importance for that player type. Forwards (numbered 1 to 8.) should have at least 100kg of weight, if not more.
Number 2 - Hooker
THe hooker is one of the most important positions in the game, he throws in the lineout, and complements your front row when in a scrum. That said, if he can do these two things well, then really it's not to important to have any other skills.
Key Skills : Handling, Technique, Strength
Number 4 & 5: Locks
The locks are similar to the hooker, very important in set piece plays. They are the main guys the hooker throws to, as well as providing most of the drive in the scrum.
Key Skills : Height (at least 195cm +), Strength, Handling, Jumping
Number 9 : Scrum Half
The scrum half handles the ball at most breakdowns, thus provides the team with the platform to build attacking moves off. So a key here is that he has the skills to do so. As well the scrumhalf does a lot of pick and go's, which mean at times he acts like a 9th forward. Kicking is also important as the scrumhalf may try and kick the ball away when under pressure, no matter if he has the skills to go with it or not!
Key Skills: Attack, Handling, Kicking - less extent strength.
Other positions
Number 1 and 3 : Loosehead and Tighthead props.
Very important for the scrum, they provide the foundation so the locks can push. Great props will complement the locks in the scrum.
Key Skills : Weight, Strength, Technique
Number 6 and 7 : Breakaways
Not as important in the scrum, they provide pressure on the backline after a movement. They also attend a lot of the rucks with an opportunity to turn the ball over. Can be helpful to have height to help in the lineout.
Key Skills : Technique, Defence, strength
Number 8
This role tends to be an attacking forward, it is suppose to be a guy who can link in with the backline to attack, maybe I play the wrong guy there, as mine doesn't seem to want to be involved unless he grabs the ball of the back of the scrum to dive over for a try. Either way he still provides a similar role to the breakaways in other areas of the field.
Key Skills : Technique, Defence, strength
Number 10 : Flyhalf
Similar to the scrumhalf, he sees a lot of the ball. He provides a link to the rest of the backline, but also can be a guy who will try and bit himself. Vital that he has some kicking as well, as he is quite adapt at having a shot a field goal from time to time.
Key Skills : Attack, Handling, Agility
Number 12 and 13 : Centres
The centres really build off what the scrum and fly half have given them. They are attacking players who need to be able to break the line, and also link with there wingers. Very occasionally they also are required to kick, but it's not important to scout for it.
Key Skills : Attack, Agility, Speed
Number 11 and 14 : Wingers are there to finish off attacking moves. By the time the ball has got to them, hopefully he is racing down the touchline to put the ball over the line. Speed is critical for these players. As well good defence on wingers combined with there speed, see's them very involved when a line break is made against your team. Something to keep in mind.
Key Skills : Speed, Attack, Agility
Number 15: Fullback
A good fullback should play like a flyhalf on the field, injecting himself into the attack when possible. The reality in the match engine is that he doesn't try to do this. That said, he will field a lot of kicks in general play, and also be required to kick himself from time to time. As a fullback, he is also there to stop a play when the line has been broken against your team.
Key Skills : Defence, Handling, Kicking.