I like multiclassed characters. I like them a lot. Some times a little mixing and matching is almost necessary to get that character to be exactly who you want them to be. However, sometimes it makes no sense and the character becomes nothing more than a stat sheet and trying to even envision the type of character that they are is an exercise in itself. And even more than that, where did they learn these new skills from? How does a Ranger, who has spent the last month in the woods hunting down and killing orcs with a bow, suddenly have the ability to cast arcane magic?
I think that multiclassing to another base class should have some requirements that go along with it, much like gaining entry into a prestige class. I don’t think this just for the sake of being a ball busting DM, but rather to make sure that the characters that inhabit the world that I create are realistic, for my sake as well as the player’s. In the afore mentioned example the Ranger is probably interested in becoming an Arcane Archer and needs the arcane magic requirement to get access to it. But how did they just learn to cast spells when nothing in their background indicates it? It would make more sense if they had at least a couple of ranks in Spellcraft and Knowledge (Arcana). Have they even read a spellbook before? The real world equivalent would be a pilot waking up one day and, not only deciding that they want to be a banker, having the skills to be a banker. Even though nothing in the background of the pilot indicates an interest or talent for the new profession, they suddenly have the ability to perform the banking job at some level. Wouldn’t it make more sense if you found out that the pilot had been studying banking between flights? Maybe took some classes? I think a couple of ranks in some class appropriate skills is perfectly acceptable, and maybe even a feat or two. Want to dip into fighter for two levels? Fine, but your character has been a wizard for four levels and needs to demonstrate some level of competence in the form of a martial proficiency or two. It’s assumed that first level characters have had some training in their respective class (to what degree varies a lot depending on the class), so it is unrealistic to think that just because someone has gained a couple of levels as a thief they can translate that knowledge into being a bard.
Additionally, by limiting how easy it is to pick up new classes some of the bizarre and overpowered class combinations are a little more difficult to make work. And it also allows some classes to mesh well with each other, which makes sense. If a ranger wants to multiclass with a scout it shouldn’t be that hard, those two classes have a lot in common. Any skill rank requirements are probably going to be easy for the ranger to meet. However a barbarian who wants to be a sorcerer is going to have a harder time, as they should. The requirements for the sorcerer are probably going to be cross class skills for the barbarian. They might just have to wait a little bit longer to be a rage mage. I would hesitate to spell out specific requirements for each class, they should probably be talked over between DM and player for each case. If they are all skill based that would put fighters and their two skill points a level at a huge disadvantage. They could even just be based on some roleplaying stuff like an apprenticeship or some time with the party mage or whatever.
I think in the end the result of this is characters that make more sense and seem more natural. I hate when I read about a build that seems real powerful, but is coming out of left field as far as flavor is concerned. Where did your thri-keen swordsage learn arcane magic?