Could Willie have played as anonymous a semi...

 Mozart
I think that the Brumbies made sure that whenever a ball is kicked to be fielded by Folau he was under pressure and not being able to start running in space.   He does not seem to be a fast runner off the mark - but excell once he has picked up pace and is obviously a very strong ball carrier and strong in evading tackles as well.
I think he has less ball sense than players like Smith and Le Roux has - but still a very dangerous player to contend with.   
Off the mark he is definitely slower in pace than Smith. Le Roux and Dagg are.    If I have to make a call - I would say that the more dangerous full backs to contend with are  in fact Smith and Le Roux - with Le Roux more unpredictable than Smith.   Smith is more speedy - but less likely to assist in scoring of tries through unpredictable means.
In 2012 Dagg had a stellar year - since then he has gone backwards in performance.  The reason for that is that players gets "worked out" by opposing teams  and this is where unpredictability plays a major role.   It is more difficult to deal with an unpredictable player than a predictable one - he is more difficult to "work out".
From a more "dangerous" point of view - I would say that the order of full backs are Le Roux, Smith, Folau and Dagg.  

 Dagg was never the same after a serious leg injust (I think it was in 2012). It was a freak injury involving his thigh muscle if I remember correctly. He came back really quickly but has never been the same.

 

He is starting to look good now though.

 Falau is a loafer and if he worked hard would be the best of them all. Coach needs to address this for the final as a ma tter of urgency.
I also think Potgieter must move to 7 and Skelton must start. Then you have a pack to be reckoned with.

 Skelton is a real menace, pity his air-supply is limited and he tires quickly and then becomes a stumbling giant.  He remains a good impact player though.

Willie le Roux is not that good.  He makes silly mistakes and is a tad on the light side.

If Willie is "not that good", I'd love to hear your definition of a good rugby player. On the light side? Would you say Christian Cullen was also on the light side at 5kgs lighter than Le Roux? As long as he makes his tackles, fields the high balls (which he does as well as anyone in the game right now) and creates tries I don't see what his weight has to do with anything. Yes, he does something silly now and then, mostly when he gets too ambitious. He also sets up and scores a lot of tries. What you gonna do?

I'm not sure if you're a South African. If so, I'd find it amusing to see you put down our best attacking weapon since Habana in his prime on the basis of his size or the odd slip-up. Me, I'm counting our blessings.

 Yes always count your blessings. It comes to mind we havent seen rooitwit for a while.
A Bok backline to tear the wee abs apart.
Reinaach, Pollard, Habana, Jean, Allende, Hendricks and Willie.
Put behind a dominant pack - has to exclude wee victor, and we should see some very impressive tries.