Lets face it, the early editions of Dungeons & Dragons did not handle the multiclass rules well. In 1st and 2nd Edition D&D only the demi-humans could multiclass. Each race had a limited number of classes they could pick between. These classes were chosen in a manner that was frankly quite arbitrary. The character would then split his XP between the two classes.
To make matters worse, demi-humans had to contend with (once again) pretty arbitrary level limits. For some reason, every race had unlimited advancement as thieves, but substantial level limits (i.e., a max level you could reach) on even classes that seemed a natural fit (e.g., Dwarven Fighters, Elven Magic-Users).
Under this paradigm, if you were playing a Halfling Fighter/Thief, you would cease advancing as a fighter at level 4, but half your XP would go towards fighter for the rest of your career. Of course, you were probably still better off than the poor sap playing a single class Halfling fighter stuck at level 4, but it was still annoying.
So when 3rd Edition came out with its new race and multiclass rules it was a breath of fresh air. Rather than being a special ability of the non-human races, anyone could multiclass. Instead of splitting a character's XP between two classes, you merely chose what class you wanted to advance in each time you leveled up. There were still some holdovers from the 2nd Edition mindset, like favored races and XP penalties. But it was a quantum leap forward.
It was such a leap forward that it took awhile to see the flaws in the system. It was usually detrimental to multiclass much if you were a primary spell-caster, since your caster level and spell-casting list were too heavily impacted. Multiclassing could also affect your base attack bonus and saving throws in unpredictable ways. Sometimes you could end up with a base attack and saving throws way below what you would expect for your level. Conversely, you could end up with a saving throw bonus way above what is normal for your level because of the +2 bonus most classes get to their primary save.
A less obvious effect was it encouraged designers to spread out the powers of a class to discourage dipping. If you made too many of the classes abilities available at first level (e.g., the 3.0 Ranger), it encouraged players to take a single level in the class just to pick up those abilities. Unfortunately, this often meant that powers essential to the class concept had to be delayed until later levels or scaled by class level. An example of the latter is the Duelist's Canny Defense which gave you a bonus to AC equal to your Intelligence bonus when unarmored but limited to a maximum of your Duelist class level.
Nevertheless, the 3rd Edition multiclass rules were a vast improvement over what had come before. So I was a little surprised when I found out that they threw them out and started from scratch for 4th Edition.
In 4th Edition, multiclassing is now governed by feats. If you are a wizard who wants to study swordplay, you can take a feat which gives you a couple of basic fighter abilities. If you decide you want to do more than dabble, additional feats can allow you to swap out some of your wizardly powers for the martial powers of the fighter. Despite this side focus, you will continue to advance as a wizard.
Interestingly, this makes the class you pick at first level more important than ever, since you will never truly leave it behind. If you are a rogue at level one, you will be a rogue at level thirty. Granted, you may have some spell-casting you picked up along the way and some special abilities granted by your paragon path and epic destinies. At your core though, you are a rogue.
So, what do I think of the change? Well, I have more reservations about this change than many of the ones coming up in 4th Edition. I do understand the need to fix the issues with the multiclass and prestige class rules. After all, I have been exposed to the horrors of the Character Optimization forums and I have to admit that the new system seems solid at first glance.
It's the IT guy in me that is giving me pause. When you have a quality product, I would rather see an attempt to fix the bugs than throwing out the code and starting from scratch.
I probably shouldn't talk though. My ultimate solution to the multiclass problem would be even more radical. I would throw out the concept of class entirely. Since they have already standardized base attack and saving throw progression, it would take very little to standardize hit points and merely provide a listing of powers that you could pick up as you level. You could label these powers as belonging to Arcane, Divine, Martial, and Thievery sets. It might be advantageous to pick powers from the same set because they would build off one another, but nothing would prevent you from making a "fighter" with a few spells, or a "rogue" who had found religion.
I have to admit that removing class would probably be just a bit too much change considering everything else that is changing in 4th Edition. Maybe that's something we can look forward to in 5th?