I don't see the issue of stand-by lines being a major issue when the parks get really busy. If anything, the stand-by lines have actually been a bit shorter.
Universal is currently dealing with the same challenges that Disney did when they introduced Fastpass. In that, with more guests using the virtual queue, rather than the "normal" queues, the streets are alot more crowded as people are waiting for their return times. Obviously, all those guests who would normally be waiting in line, are now shopping, eating, and wandering around the park. This makes the park seem more crowded than it actually is. Disneyland during this past Christmas is a great example of this phenomenon. Virtual queues can actually lower a park's physical capacity.
Look for ALOT more street entertainment to start popping up around the theme parks to help give our guests something to do while they are "waiting."
In terms of Spring Break:
The only thing we've seen so far is that, when the park gets busy, those Express tickets start running out FAST. Typically, the only attractions that DON'T run out of Express tickets are: Storm, Bluto's, Trike, Unicorn, and all of Seuss.
In my opinion, the 3 Express tickets to get first are: Hulk, Spiderman, and Poseidon. These 3 always seem to run out first (around 1pm)