| Arrived from Barcelona and cleared customs with sufficient time to get aboard an earlier departure flight to my next destination. The Departures sign board indicated the gate as D 9 (if memory serves), so we quickly followed the signs towards corridor D. But guess what, even though one hallway was clearly marked with a D (and an arrow) it had a barrier across the entrance and a big floor sign: No Admittance. So back we went to the main corridor, where we saw directional arrows for Gates D-E etc, so down that corridor we race, looking for an American Rebooking desk. None in sight, anywhere, despite this being a major American hub. Stopped at one gate that was staffed and asked where the rebooking desk was, and was told it was something like D-37 or D-3, and then the agent volunteered that one might be a little closer, so off in that direction we ran. When we finally got there, we were told that the doors had already been closed (never mind that it would have been about a 15 minute walk to get there anyway). So we resigned ourselves to an interminable wait down near D-1, where a bottled beer was "only" $10.50, not counting tax and a "suggested tip of $1.50." When it finally was near time for the departure for the later flight, suddenly there was a flurry of activity at the adjacent gates, when apparently four American flights were to depart within 15 minutes of one another from gates D-1 to D-5. And since there seemed to be only one PA, the assembled passengers became understandably restless when the gate attendant announced "we are now boarding Groups 1 and 2" without bothering to state the destination or flight number. Confusing? You bet. Especially when said announcement was drowned out by another PA telling "passenger XXX, please report to the gate." Final boarding began when five wheelchairs were rolled to the plane, followed very quickly by anyone who simply showed his/her boarding pass and walked aboard. There was only one American gate agent on duty, apparently all the others were still dealing with the adjacent flights. Not professional at all.