How completely can buses come to resemble streetcars/trams? Paris has crossed one important line that North America still seems stuck on: off-board fare collection. Have another look at a new Paris bus:
Just forward of the back door, just above the advertising panel, is a green dot. It's a button that opens the door from the outside, exactly like the buttons on Paris trams. You can board a Paris bus at any door, unless you need to pay cash, in which case you board at the front. If you board at the back, machines are just inside the door to either tag-on with your Navigo smartcard or validate your magnetic stripe ticket. This is a standard proof-of-payment system, enforced by roving inspectors, just like what North Americans see on light rail but never on buses.
If you're used to dealing with crowded buses in any North American or Australasian city, you can probably sense how transformative this would be. First of all, if a bus is mostly loading (e.g. at the beginning of the line, where this bus is waiting to start), people can board all doors, so if there are three doors they can board in 1/3 the time. Second, front-door collection forces people to start at the front of the bus and then move back, and this movement itself takes some capacity. On Paris buses, there's no need to move around in the bus, instead, people tend to stay around the door where they boarded, just as they would on a tram or subway.
So these buses are better for both capacity and comfort. Capacity, because the need for people to move around in the bus takes some space. Comfort, because while standing may not be ideal, fighting your way through other standees to get to the door is much worse. Note, too, that the whole North American crowded bus ritual, where the driver is constantly saying "please move to the back," just doesn't happen, so there's one less opportunity for the customers to feel like cattle.
In the middle of the line, where you have people getting on and off, these two movements obviously conflict on each door. This looks pretty inefficient from the outside of the bus ...
... but it's still more efficient than forcing people to move forward or back through the bus as other fare collection systems do. After all, in a Paris 3-door bus, this situation is happening at all three doors, and if one door is moving better people will shift to it. The result is a process that naturally optimises the flow through all three doors, and thus minimizes the dwell time.
(Observing the situations, I wondered if it would ever be possible to train people to keep right as they enter and exit the bus. There is a small railing that divides the boarding path into two streams, and if these were used for opposite directions of traffic (at intermediate stops where both directions exist) this situation would move a little better.)
So why, exactly, is all-door boarding on buses so hard in North America and Australasia? The fare transaction experience can do a lot to form impressions of buses vs light rail. Nobody really likes that moment of being judged by the driver as they board, or the boarding queues that this moment requires. So if you can do proof-of-payment on light rail, why can't you do it on high-volume buses?