This topic has been debated to death in the past - but hey some of you are new (maybe even too young to remember Subang airport)...
1. Kuala Lumpur was in DIRE need of a new airport in the early 1990s as Subang was limited by being located in an area which was becoming increasingly urban.
2. Subang's location meant it was limited to ONLY having one runway. Passenger terminal space was also limited eventhough Terminal 1 was expanded in the 1980s to its maximum possible size. Terminal 2 (initially built as an interim terminal whilst T1 was rebuilt) HAD to be reopened to off-load some of the squeeze by shifting the busy KL-Singapore shuttle service there. Terminal 3 was finally, hastily built at the other end of the runway to cope with the increasing demand for space at T1 in the early 1990s.
3. During the 1990s, KUL saw a surge in passenger numbers and foreign carriers. UTA (and later Air France), Kuwait Airways, Ansett Australia, Air New Zealand, China Southern, Eva Air, Balkan, All Nippon Airways, MEA, Iran Air, Merpati, Vietnam Airlines, Royal Air Cambodge, Myanma Airways, Gulf Air, LaudaAir, Indian Airlines, AirLanka, Nippon Cargo, Emirates, Saudia, PIA, Fedex and Uzbekistan Airways ALL began flights during this period.
4. British Airways, China Airlines, Cathay Pacific, Lufthansa, JAL, (even) SIA, Thai, Biman, Garuda and KLM ALL boosted services into KUL, whilst KUL's long time foe Qantas reinstated a 747-300 service into Sydney. Believe it or not - British Airways at this time complained of being unable to gain favourable slot-times!
5. MAS was riding high with new destinations like Mexico City and Buenos Aires whilst doubling services into key markets like Australia and Europe. Other carriers were keen to come into KUL like Virgin Atlantic, Swissair, Alitalia, Egyptair and did so by joining MAS in the (then) 'newish' concept of code-sharing. Aeromexico, South African Airways, (now defunct) Zambia Airways, SAS, LTU, Varig, Asiana, United and Northwest all began looking at KUL as a destination.
6. Car-parking became an IMPOSSIBILITY - cars were regularly seen parking up to 1/2 mile away from T1 down the Airport Highway.
7. Check-in became a NIGHTMARE. BA and KLM used to notify passengers to check-in up to 3 hours in advance for fears of huge queues - as T1 only had 60 check-in desks - of which 30 were for MAS only. BA and MAS started trying out city check-ins to try and cope with the lack of space.
8. Gates were often not assigned until boarding time as the airport layout made it almost impossible to predict which gates would be available. Passengers often had to trek to one end only to find that a previous flight was delayed and their aircraft was now docked at another gate or even worse - at the Cargo Centre which was located on the other side of the airport, across the airbridge - accessible only by airport buses which constantly stopped to let aircraft pass.
9. Duty free shops were pokey and there were limited Food and Beverage facilities and toilets or lounge facilities.
10. Aircraft often had to wait in long queues after landing or before take-off as the single runway only had one taxiway. Efforts to build a second taxiway did not help to relieve the congestion as the parking bays were always full. I once waited on a BA 747-400 for 25 minutes by Terminal 3 before the flow of aircraft on the ground made it possible to park at T1.
11. T1 queues on arrival at Immigration were notorious due to the shortfall in desks available. It had only 4 baggage carousels - which were always overflowing with luggage - handlers had to be employed to constantly take pieces off and stack them in rows by the carousels to create more room.
12. The arrivals hall had its 'Pick-Up' point removed due to the lack of pedestrian space - passengers then had to lug all their luggage up a long flight of stairs towards the car park.
13. Technologically - the ATC was outdating fast. Morale was poor - a fire which was set by an arsonist closed the airport for 3 hours - whilst I circled over Sumatra in a Qantas 767. Many other fires occured due to old wiring.
14. T2 became much like a bus stop for Shuttle users - there were hardly any facilities as there was no room. SQ was NOT happy. Passengers were even less impressed.
15. T3 injected some new life to a dying 'grande dame' but by now there really was NO space left - thank GOD for the new Kuala Lumpur International Airport - which has now made the whole idea of flying into KL so much more pleasant. It deserved to rank no.2 in the world and once the Express Train starts next Spring - NO-ONE will be able to complain of its distance....
...now all we have to do is erase the fact that the economies of Asia crashed shortly after the new airport opened. Now that was hardly an idea the government, architects, developers and investors (and that includes all airlines concerned) had in mind.