I think one of the most confusing things for people is the word "mood". Everyone has moods, right? So it becomes a matter of range. But for BP it's more complicated. It's also in how the energy levels are so extreme in their shifts.

I'm going to be lazy and link to something I wrote before about the energy shift aspect:
http://forums.psychcentral.com/3655841-post11.html

And I'd add to that duration.

Oh! The other BIG thing is that for BPD, big mood shifts are precipitated by situational things. They don't come out of nowhere. (Though it can seem that way to those around them -- at least, that is, till one learns to tune in to how the BPD person hears and interprets words and actions/inactions). This isn't a sometimes thing (like it might happen in BPers), but a consistent pattern. Situation> react. Situation>react. Situation>react.

The other real biggie is the idealization/devaluation switching thing. I know that'd be something listed in the criteria, but it might be worth reiterating in contrasting them, because it's very distinctive and pretty easy to discern.

There's a good description of this right here on PC (under Conditons>BPD):

Unstable and intense relationships.
People with borderline personality disorder may idealize potential caregivers or lovers at the first or second meeting, demand to spend a lot of time together, and share the most intimate details early in a relationship. However, they may switch quickly from idealizing other people to devaluing them, feeling that the other person does not care enough, does not give enough, is not “there” enough. These individuals can empathize with and nurture other people, but only with the expectation that the other person will “be there” in return to meet their own needs on demand. These individuals are prone to sudden and dramatic shifts in their view of others, who may alternately be seen as beneficient supports or as cruelly punitive. Such shifts other reflect disillusionment with a caregiver whose nurturing qualities had been idealized or whose rejection or abandonment is expected.

I got a late start today and need to get some stuff done right now, but am happy to help out more later if you would like. I really don't think they're nearly as hard to tell apart as is commonly thought. (Me: BP, BF: BPD… Because we're each solidly and clearly within our respective camps, maybe it's the vantage point. Nothing like almost 5 years of 24/7 observation/experience to be able to see it, lol. I'm not saying everything's cookie-cutter. We're representations of each, but not THE representation of either. Still, it was quite awhile before knowing that he was, so the "AHA!" moment clarified those aspects of behavior I could not understand. Which, as it turned out, are the differences between them.)