One of the things that tends to happen as times goes by and new editions of the same game are published is that we forget quirky, older rules and how they actually worked. One of those rules is multiclassing. In D&D 3.0 and later editions, multiclassing was possible every time a character leveled up. At that point, he could choose whether to remain in his current class or change to a new class. Furthermore, there were no impositions on which classes he could change to, except the favored class rule which was as arbitrary as any other rule I have seen, forcing an experience point penalty if none of the characters were your favored. Not so in older editions and here I'm strictly speaking of AD&D 1st and 2nd editions.
In those games, multiclassing had to be a demi-human and choose to be multiclass during character creation. He could not start with just one class and choose to multiclass later. The list describing which classes could be combined was limited, too. For instance, an elf could only multiclass as Fighter/Mage; Fighter/Thief; Mage/Thief and Fighter/Mage/Thief. Humans, on the other hand, could be dual class but the limitations imposed were even higher, with certain minimum attribute values of the class he wanted to change to. An additional factor was that all experience gained was equally divided by the number of classes the character had, which meant his progress was slower than other characters.
As I continue my journey of discovery (or rediscovery in the case of AD&D 2nd edition), I constantly run into these quirky little rules that I had completely forgotten. And they work really well in the context of gaming. One of my players was always ecstactic when we played Dark Sun and he could be an elf mage/fighter/cleric/psionicist, which was a special combination only available in Athas. This would impose a greater burden on him since he had to effectively level four classes at the same time, spreading experience by four, but it was great fun as he began playing four different classes with four different sets of powers. And this, more than anything else, more than made up for any disadvantages that multiclassing could bring to the table. In the long run, an elf could reach level 12 as a mage and level 15 as a fighter (if multiclassing Mage/Fighter) whereas a D&D 3.0 could only reach level 10 in both classes.
On a more personal note, I love these tidbits. Though, according to current standards, a player may be forced "to work" harder to level his character. Then again, with each succeeding edition, there is a tendency to apply changes in the name of fun and playability. In AD&D 2nd edition, which was the one I played most, I can't think of a single moment when the rules got in the way of fun.