Airline travel is more complicated today than it has been in a long time. As airlines try to increase profits by being more efficient for themselves that often translates to being more complicated for the customer. There are also a lot of new security precautions being taken that can make flying more of a hassle and these precautions make it more important to be prepared Before You Go to the airport.
What to Pack
With more airlines charging fees for checked baggage (see a list of fees here), and tighter security on carry on luggage, you have to be very careful how you pack or it could end up costing you a lot of money.
Connections
A big part of airline efficiency is the use of hubs. That means that each airline has a few airports where most of their flights are funneled through and there are few direct flights from your home airport to your destination, unless you happen to live by a hub. Most flights will require at least one plane change so it's a good idea to make sure you have enough time between your flights to catch the next flight. Also keep an eye on the airports. I was flew through New York, but I flew into one airport and my connecting flight was at a different airport. I had to take a taxi for 40 minutes just to catch my flight even though it was booked in a single ticket.