The common factor among all UK PhDs I know of is that there is a rigorous viva at the end, where the outcome for the sudent is not a foregone conclusion. Despite the shorter overall duration of the PhD (~3 to 4 years typically), the oral examination can (rightly) cover material far beyond the scope of your thesis - if you are an expert in your field, you will be able to have a knowledgeable and informed discussion as a peer with your external examiner, who will be a recognised expert in the field. I'm definitely a big believer in the importance of being able to have a well-informed discussion around the area of your work, and actually found the whole viva process very enjoyable and cordial - a nice chat about the wider field, my and the examiners' own previous work, some debate of the merits of different approaches, and then onto a run-through of the thesis, chapter-by-chapter, skipping any pages where there were no points for discussion or contention.
Unlike European vivas though, there's no family or friends, no champagne corks being popped mid-defense, and no foregone conclusion of the outcome. I've been at European vivas with the family of the candidate preparing the celebratory buffet at the back of the auditorium while the questioning continues!
While the UK model is probably preferable in a lot of ways, this sounds much more enjoyable :)
At least that's the theory - I don't know if in practice it holds up; most of the grad students and later I knew from that system came from oxbridge which has a number of quirks.
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.
My impression was that someone failing their viva was seen as a serious failure on the part of their supervisor, who should not have let them submit without being more prepared.
That matches my limited experience with UK, although I've seen north american ones without a foregone conclusion - seems really dependent on institution.
> The most common way to "fail" is to simply not submit the thesis.
I think that's true everywhere, lot's of people just don't complete. I've heard somewhere that the "ABD" (all but dissertation) rate in US is nearly 50% at 10 years, if you go across all disciplines.