The daypass thing is mildly more confusing, but I suspect part of the logic is to encourage use of the Binge-On technology, without which towers are likely to get clogged pretty quickly. I also suspect that the soft limit of 27G a month will be torn through pretty quickly by anyone making heavy use of HD video. Go over the 27G and you're "deprioritized" - you'll get full service during quiet times, but you'll be throttled when everyone else is trying to use the network (which is fair, but you probably don't want it to happen to you!)
The big improvement is that Tethering is now an acceptable half-megabit/s, rather than 2G speeds. That makes "Unlimited tethering" actually useful again.
The big question for me is how to encourage video streaming companies to sign up to Binge-On if there's no incentive. In theory, they can just transmit 1080p over HTTPS (protocols like DASH are HTTPS friendly) and T-Mobile will never know.
With the original implementation, the advantage was that your viewers could watch your services without worrying about it coming out of their data. But if data is unlimited...