If you're not carrying on your bag, you won't want to use it for your valuables, your fragiles, or your photographic items. I carry my passport and tickets in a holder I wear around my neck through the airport.
Remember to take enough clothing in your carry-on to get you through at least a day. They do lose luggage, although usually for a very short time. My luggage seems to suffer a failure rate of around 20%. I've never waited more than a day to get it back. It's good to have a spare shirt at least while your bag is in Timbucktoo and you're in Paris. The one you slept in on the plane probably isn't appropriate for business haggling.
And that's about it. If you're going in winter and need a heavy jacket or sweater you might consider a compressor bag to drastically reduce the size of your winter wear by sucking the air out of it.
Oh, and one last suitcase tip: Don't buy black. Honestly. Black is so popular I once spent 45 minutes looking for my mother's bag at SFO because 90 percent of the bags looked exactly the same. And the same was black...and wheeled.
