Hiya!
I think multi-classing is fun and interesting...in 1e and 2e AD&D. Everything after 2e, however, sucks.
It's not that I think they are more powerful or anything (although some combo's, when also mixed with other optional rules like feats, spells, races, etc., can produce some monstrosities). It's that someone who has "Fighter/Wizard/Thief" on their character sheet isn't actually a multi-classed F/W/T...they are a character that is a Fighter, and a Wizard, and a Thief.
My problem, if I boil it down as much as possible, is that each class sits COMPLETELY interdependently of the others...mostly (mechanics wise) with regards to XP progression. When that F/W/T gets 1400 xp and gains a 'level', they up *A* class or add a new one. That screams, to me again (ymmv), "You just gained a level of Fighter! ...the last three months of sea travel, fighting the leviathan, rescuing the sea-princes bride to be, sending the horrible water-demon back to the abyss, sneaking into the half-submerged lighthouse dungeon, and deciphering all the runes, puzzles and riddles had ZERO EFFECT on your capabilities as a Wizard or Thief". All the sneaking? Irrelevant. All the Sneak Attacks? Irrelevant. All the spells cast? Irrelevant.
So, in my mind, the old-skool way of actually BEING a Fighter/Magic-User/Thief from day one, and advancing each class more or less collectively, has a completely different feel and in-campaign narrative than the 3.x+ versions of the game where you are but a single class at level 1. Then you add a new class later. Then maybe another after that. At no point are you ever "advancing all aspects of your skill-set" at the same time. Ever. It's only ONE at a time. Always. That feels completely different than the 1e/2e characters that are multi-classed. Again, IMNSHO, 1e/2e did it MUCH better. Like, leaps and bounds better.
^_^
Paul L. Ming