To be successful, I think that the public transport system must not be crowded at the start.
To be frank, there are a lot of buses in SLC that aren't particularly crowded. Basically, ever. Some even approach the dreaded 'one rider per bus' status, during off-peak hours. One thing I saw in DC that I really liked was peak-hour pricing. It is noticeably more expensive to ride the metro during peak hours (150%-ish?).
Mike Brown has often advocated making transit free, in order to increase ridership, on the basis that the farebox really doesn't provide that much revenue for the transit agency, and that the time penalty for fare collection during boarding is significant. I'd say that Eugene's Emerald Express (EMX) validates that. Making the bus free cuts down on boarding time in two ways--directly, by eliminating the time necessary to collect the fair, and indirectly, by enabling multi-door boarding. Most buses have two doors as a standard feature, but most systems only make use of the forward door, where the driver can prevent fare evasion. Using the back door to unload is out of the question, because it would permit ingress as well as egress. So first everyone has to get off the bus, and then everyone can get on the bus.
With no fare, it no longer matters. The three door Van-Hool buses UTA uses for the 3500 BRT take this to a whole other level.
I've argued that some fare is necessary for people to 'keep some skin in the game', and avoid the over-use of a common property resource. Increasingly, I realize that my argument is bunk. While that might be true at a later time, or during the peak hour, when buses are crowded, it's certainly not true for the majority of the day.
So it may behoove UTA to start running 'free buses' during off-peak hours. It's not going to significantly affect their farebox ratio significantly, and they're going to keep running the buses anyway, out of political pressure, so they might as well try to get as many people as possible on them while they can.