Multiclassing in 4th Edition has gotten a lot of flak since the specific mechanics were introduced a short time prior to the game's release, probably because it functions nothing like it did in the past, excepting that it lets you pilfer abilities from a class other than the one you started with. Some people think that it sucks just because of this, while others get all butt-hurt because they think that either: A) multiclassing feats suck too much to be worth the effort, or B) that it just doesnt let you spread yourself out thin enough.
In general I don't think that is a bad thing. In fact, I like it more than before since it makes more sense when you stop and think about it.
Huh?? Yeah. WHA?? Uh-huh! Explanations abound!
In 3rd Edition class wasn't a hardwired option for you, as it was really a point based system masquerading as a class based game for the sake of tradition. At any point in your character's career, you could opt to pick any class you wanted to when you leveled. ANY. You could start as a fighter and then take a level in wizard, rogue, dread Necromancer, totemist, whatever the fuck you wanted. I'm not going to go into the logistics behind this, but suffice to say that problems would ensue depending on what you did since many classes had abilities that scaled by level. For example, there was little point in multiclassing into wizard for only a level or two since the more levels you got the shittier those spells were.
In a nutshell, abilities and features tied to a specific class's level do not work when you can freely pull levels from multiple sources on a whim. Well, they work so long as you never stray from the class they are tied to.
The other flaw isn't really mechanical but...not...mechanical...? Er, it arises when you consider the game narrative/consistency. What I mean by this is lets say that you started out the game as a wizard. You're proficient with only a couple simple weapons and no armor of any sort. You go adventuring for a bit, perhaps a few days, and gain a level. Usually you'd just take wizard again so that you get more spells, but you could instead take a level in another class. Lets say...fighter. Since you gain all the class features, proficiencies, and class skills of the class, your wizard can now use all simple and martial weapons, as well as all forms of armor without any penalty whatsoever (except for spell failure, that is). Make sense? Yeah, I thought not.
For better or for worse, 4th Edition does not roll that way.
Multiclassing in 4E is a mechanical option that lets you acquire powers and/or class features from a class other than the one you started as by expending feats, but you have to burn a feat each for encounter powers, daily powers, and utility powers. What this means is that you end up "dabbling" in another class. Using the above example you dont gain a level and spontaneously gain a mastery of all cantrips and low-level magic, you instead pick up a single spell and also a fundamental understanding of arcane magic and shit. As you get higher level you can gradually pick up more and more spells at the expense of learning whatever the hell it was you were doing with your actual class.
The best part is that its considerably easier to ensure that your attacks are effective since A) attack bonuses scale by level, not by class, and B) powers also scale and you can swap them out as you progress. So, no having to fall back on magic missile at epic tier. The only drawback that I've encountered is that the ability score used doesnt change, so in some cases you might have to stretch your ability scores a bit in order to make it work.
As far as feats go, the opening multiclassing feats are actually quite good for what they provide, which is basically Skill Training plus something else. Generally if I want to pick up Skill Training I end up browsing the list of MC feats to see if one will give me the skill I want. Past that things get a bit trickier because you have to burn a feat each if you want to swap out encounter, daily, and utility powers. I think people get hung up on this "feat tax".
On one hand I want to say that it does seem a bit much since you are just swapping things (and therefore gaining nothing), but on the other hand you get twice as many feats as you did in other editions and its not like any particular feats are necessary to survive. Plus, you can always paragon multiclass and not pay anything at all for your troubles. Or both. Whatever.
Actually, what I would like to see are multiclass combination feats that let some races get extra benefits when they multiclass in a thematic combination, kind of like how tieflings got that infernal captain thing going on, just not as shitty (warlords and Con-based warlocks dont really mix).
From a narrative stance this method shines since you dont get an entire class's suite of stuff, just a trick or two. It makes so much more sense that the Fighter was able to pick up a spell by "reading over a Wizard's shoulder", instead of somehow mastering them all in a fraction of the time it took the wizard to do so.
I'm not an optimizer in the sense that I methodically construct characters, mapping them out in intricate detail throughout all 30 levels. Mainly I think of a concept and just roll from there, preferring to take the "organic" approach and pick new options as I actively play the character and see how it grows. Usually the starting concept is a logical one, such as a Warforged Fighter or Longtooth Shifter Barbarian. Sometimes its a bit more unorthodox, like a Gnoll Monk or Gnome Barbarian. Depends on if the concept looks cool in my head.
The entire process goes like this, I pick a race like...minotaur, and mash it with a class like, say...rogue? Well they're strong so I'll go with the brute scoundrel class feature and try and pick powers that emphasize mobility and forced movement. Is this ideal? I dunno, just sounds cool to me. I then go through the motions picking feats and powers that work with the concept. They may not be the most optimal, but they're the ones that fit the theme that I'm aiming for.
Multiclassing can help with this conceptual phase quite a bit, and I dont have to worry about it crippling me in the process. I made a Minotaur Tactical Warlord/Artificer that was based on the idea that he was a general during the Last War and got hit by a particularly nasty living fireball, which caused him to lose his arm. He got a mechanical replacement, and decided to learn how to maintain it himself. This concept was made functional because of how multiclassing works in this edition. In 3rd Edition it would have made for a very sub-par character since my Artificer infusions would have gradually depreciated (and my attack bonus would have sucked ass, making for a very piss-poor Fighter...which is saying a lot since 3E Fighters were always piss-poor).
So thats my opinion of multiclassing. It works out alright from a mechanical and narrative perspective, perhaps better on the latter. I think a good "fix" would be to make racial feats that make it easier to pick up powers from thematic classes (like eladrin multiclassing into wizards, elves into ranger, etc). Maybe a feat that lets you use another ability score for attacks instead of the original one? Dunno. Really though I'm content with it as is since for me multiclassing is an option to assist me in realizing a concept, nothing something I utilize for min/maxing purposes.
Next up, hybrid classes.