What Cassar's appointment, and Kreis' departure, mean for RSL is probably better discussed by RSL fans (SBNation's own Matt Montgomery goes long on it here) the basic takeaway is that the Utah side wants to keep it in-house. Cassar has been with the team since 2007. Robin Fraser, reportedly the other finalist for the head coach position, started the same year, but left for the head coaching gig with Chivas USA in 2010.
As for the Red Bulls-centric take: Cassar's promotion means Fraser can stick around as Mike Petke's deputy going forward. Yeah, Petke's getting an education from some of the English Premier League's best, but it always helps to have a second opinion from someone who's been there before. While Fraser didn't put up the best record at Chivas USA, it's important to note that it's Chivas USA.
There's a good chance Fraser will get an offer from someone, somewhere before too long, but it's good that he's sticking around for now. The more people Petke can turn to as his coaching career progresses, the better.