Quote Originally Posted by Demonic_Duck View Post
There are many idiomatic phrases in Russian, just as in English. Compare «я хотел(а) бы» - "I would like", which also uses the past tense (even though the desire it expresses is in the present).
I think, it is a way to say rather than idiomatic phrase, because any verb can be used instead of "поехали" with the same sense "let's do it right now".

Quote Originally Posted by Demonic_Duck View Post
In the case of «поехали», I would guess it adds a sense of urgency (let's be gone from here sooner rather than later), but I could be wrong about this.
Yes, something like that. It is like an imperative cooperative ("let us do it") with the idea of immediateness ("right now").

- Едемте в лес! (imperative cooperative)
- Едемте.
- Ну так поехали!

- Let's go to the forest!
- Let's go.
- So, let's go right now!