ive been involved with rugby in the US, and admittedly i was surprised at the awareness and level at which rugby union was played at... Rugby Union actually has a very good reputation in the US, people in the US aren't at all concerned by the stoppage in play for things like scrums or lineouts as this is all to familiar to them due to the NFL, and there is a certain level of respect for people who play the sport due to the lack of protection they play in.
Physically the US is inundated with highly talented athletes, from 6'4" 115kg men to your 95kg wingers who can run 100m in 10.8sec, there is some exceptional athletic talent out there waiting to be tapped into. Even with the other professional codes picking off the top athletes, there is still a fair bit of room for rugby union to pick up there fair share.
However, there are some issues that really need to be addressed in coming years, the main issue been the coaching skills and technical understanding of the game which is very poor, the reasons for this are numerous but for the standard of rugby union in the US to improve, they really need to invest money in coaching clinics. They might eventually get more numbers, and have athletically gifted players suitable to the 7's version of the game, but until they actually get some technical nous of rucks, mauls, scrums and lineouts filtering down through the grades into juniors, then they will continue to struggle at the 15's game.
If Rugby Union continues to rise at its current rate, then its still going to be at the very least 10years before they will start been competitive at the 15's level internationally.