Basically, they are responsible for the equipment, and you are absolutely not.  I'd bring extra headphones and needles (if you spin vinyl), and maybe a couple of RCA or XLR cables/converters if I wanted to ensure the monitor will work, but most importantly, make sure the contract is a pay-or-play, and they pay up front.

That means if the sound system is screwed up or the cops show up because the promoter lied about it being a permitted gig, you're still paid.

I also make it a habit to be able to play an entire set on either vinyl or CD, and I carry my CDs onto the flight instead of checking them into luggage.  This prevents a few disasters, the most common one being that they promise two Technics -1200's & two CDJ-1000s... and give you the Technics only.  If your luggage gets lost, your CDs will still allow you to play a full set.

By the way:  The paying up front thing... for me, that's non-negotiable.  If the money isn't in my bank account, I don't get on the plane.  Period.  Also, if I don't get all my plane tickets when I check in (round-trip), I don't get on the plane.  Do not give anyone leverage on you, ever.  I have seen the most shady stuff ever, here's a brief sampling:

• Getting on the plane with one ticket and without being paid, and finding yourself facing a weak explanation on "low ticket sales" and pay cuts.  (If you refuse, they'll ditch you at the airport and you'll have to buy your own ticket home.)
• Being asked to pay for your own tickets, because "the online order didn't go through," and oh yeah, sure, I'll reimburse you!
• Being promised a check once you arrive at the gig.  You may even get one, but it'll bounce harder than a superball. 
• My personal favorite: Offered weed in payment.  Hah!

This is why I absolutely require contracts whenever I could be put at risk for a lot of money.  Local gigs... I still ask for them, but if it's a favor or something I won't stress out too much.

Contracts need to be enforced, too.  If you break contract (play without being paid), the promoter can argue that he is longer bound by it.  This really, really sucks if we're talking about that elusive return-trip plane ticket.

A bunch of other industry stories, including an MC I know who was offered food stamps in payment and Steve Lawler's run-in with Mexican organized crime, make me deeply concerned about my own liability in the event of promoter shadiness.