The bolded part is to me the key difference. Once one verifies that a written account is not a later forgery, one can be certain it hasn't been changed in the decades or centuries after it was written, and one can be certain the person's writing wasn't influenced by anything that would happen later.
That's fair. Another way of saying that is that oral traditions tend to keep history alive, while in written accounts history is relegated to the past. That can be both an advantage and a disadvantage, though.
An advantage is that keeping a living history through oral tradition tends to preserve context better. Because it is kept alive through people rather than paper and ink, it often provides a greater opportunity to ask questions about it and have them answered. It's harder to have a conversation with a dusty tome.
Kuwi, it might help to elaborate on the methods used. Most people these days are unfamiliar with the particular techniques of preserving and presenting oral tradition.
Lisa mentioned a few of them, such as the ritualized practices of only telling some stories at particular times of the year or at certain places or to certain people. In Zuni, tales must be told during the winter, while true stories can be told all year round.
There is also the acknowledgement of other versions, and often this can come with an implicit understanding of how the versions relate to one another. For example, many versions of our creation story exist, but they all draw upon certain versions that are considered to be the "official" versions. In addition to humans, different kachinas tell different versions of it, and the "most official" version is the one told by Kiaklo, who is the keeper of our history. While other versions may introduce variation, this version never changes, except maybe to include recent history. (An interesting cultural note is that we don't call it a "creation" story or differentiate it from the rest of history.
Any history or "true" story can be considered to be a part of the same story, as a continuance from the beginning of time. There is no significant difference between the past of a thousand years ago and the past of yesterday. Both are equally connected to each other and to the present day and the future.)
Many oral traditions call upon poetic devices. Or rather, many of what we now consider poetic techniques can trace their origins to oral tradition. These greatly depend on the language, so it's hard to generalize. European examples can be found, for example, in Homer and Beowulf. You see the use of rhythm and meter: Homer being written in dactylic hexameter, for example.
In other traditions (depending on the language) it may be more difficult to characterize meter, but it's not uncommon for oral tradition to come in the form of song or forms that are chanted. Even when it is simply spoken, there is usually a rhythm, and everything from timing of pauses and volume become significant.
Other techniques include repetitions (saying the same thing multiple times for emphasis) and motifs of re-phrasing, such as use of synecdoche and metonymy. Repetition and re-phrasing key ideas multiple times or in multiple ways is a very common technique.
There are often ritualized beginnings and openings and transitions between sections. Homeric tradition begins with invocation of the gods or muses, for example. Likewise, Zuni stories begin and end with set phrases, which also indicate the kind of story (tale or history, etc.), and variations on these set phrases can also say something about the story. For example, one might tell a true story in the style of a tale, or a tale in the style of a true story, and this would be evident in the way it is told.
Similar to many traditions of poetry (or rather, poetry inherits these attributes), there is often an extensive lexicon of set phrases and words that represent meaning. There are metaphors and imagery that are understood to represent specific things or actions. This can be found everywhere from (relatively) modern poetry to Biblical stories to ancient oral traditions around the world, where meaning is preserved through elaborate ritual shorthand, and knowledge of the ritual language is often necessary to fully understand what is being said.