The difference I tend to see is that the outcome of the UK viva (oral) examination is far less of a foregone conclusion, and can still be quite traditional in that students are expected to be able to have a broad and well-informed discussion about their wider field and the context of their work. The thinking is that they will (if meritous of a PhD) have a certain level of expertise, and thus be able to have a discussion with their external examiner (a distinguished and recognised expert) as a peer.
My experience of it was that if you are genuinely knowledgeable and approaching being an expert in your field, it is an enjoyable experience, and just like having a (longer than usual, but not uncomfortably long) conversation with someone about a topic that you both share a deep interest in. There is nothing to worry about, as you can have a nice discussion about an interesting topic, and share interesting ideas etc.
I'm not sure if it's a lower bar for grad as such - I think it's got fewer "formal" requirements, and far more informal requirements. The most common way to "fail" is to simply not submit the thesis.