I also don't like too much the new scheme.
When I introduced the ticket types, it was mainly
to have a high-level distinction between bugs
and features requests (visible in the timeline)
and to make it possible to associate a severity to
the feature requests (e.g. trivial, minor, major, critical)…
The point of the other Trac developers was that
ticket types + severity + priority would be overkill
most of the time.
Then, several people expressed their wish to keep
that flexibility, so the compromise was to keep
the three fields but to have one of them
inactive by default for new installations.
That's why Severity is not enabled by default
and Priority has the old Severity values
(that last point is the most controversial, IMHO).
It's often said that, for instance, a critical bug
should always deserve highest prio, so having both
fields is redundant.
At least, if we don't re-enable the severity field,
the default priority values should be more intuitive
and look more like priorities:
highest, high, normal, low, lowest