The MBTA started adding some new buses to its fleet a while back. I don't think any of them are operating on the route I use most frequently, but they are running on the 101 so I've gotten to ride them several times on that route.
The most obvious and most pleasant thing about new buses is that they're clean. Buses see such heavy use that they lose their luster pretty quickly even when they are cleaned and maintained, so it's nice to be able to enjoy them at the start of their life cycle. The seats are made of a textured plastic that holds riders in place a bit better than the older buses' smooth seats. It also seems they decided to bring back the tinted glass.
I also noticed that the motors have a stop-start function, which probably helps save a little fuel. In theory these systems are great, but in usage it's a bit rough. When idling at traffic lights, the motor shuts down after a period of time, and the bus gives a big shudder. When the driver pushes the accelerator the motor restarts, and that part is a bit smoother than the shutdown but it's still slightly jarring.
It's entirely possible that implementing such a system on a vehicle as large as a bus is more difficult than adding it to a car, and I don't want to sound like I'm complaining about it. I'm not. I commend the T for choosing vehicles that will consume less fuel. The system just takes some getting used to as a passenger.