Welcome to the forums @Tim Pearson.
Although you may have only seen a rubber-belted conveyor, there are several different types out there - continuous rollers, for example.
The style of conveyor used on Thunder River is fairly standard for this type of ride, and has been in use for decades. As has been discussed ad nauseum, it is not the type of conveyor that is in issue here, but rather the modifications made to it that are suggested to have been the cause.
Take these images - from a river rapids ride at Disney's California Adventure:
In comparison, you can see the slats at dreamworld are not as closely spaced:
There are of course examples of river rapids rides that do use a rubberised belt:
As for a rollbar, most river rapids don't feature this - however, from what I understand of this accident, a rollbar likely wouldn't have prevented what occurred. the raft did not flip completely over.
There's quite a discussion on Parkz about the incident, with loads of speculation - it's worth a read if you're interested, as much of what you've pointed out has been discussed at length with good examples and arguments both for and against.