Z, you make some good points! I'm just going to cull out a few things to reply to; I think in the end we simply have different preferences for multiclassing, and I doubt whether either of us is going to convert.

Here's hoping 5e supports both of our styles!
I think you're correct.

My intention is not to convert, it's to discuss. I adore this game and enjoy talking about it. It's why I searched for and joined Eric Noah's 3rd Edition News way back in the day, why I wrote such a favorable
review of the game when it came out, and why I come back to what is now ENWorld to geek out on new edition speculation. It's been my honor and privilege to be an in-the-book credited playtester for 3rd, 4th, and (assumedly) now 5th. And of course that same honor and privilege to argue design merits with knowledgeable enthusiasts such as yourself.
the Jester said:
Interesting. My experience- dating back to BECMI/1e days- is exactly the opposite; probably around 1/3 of all the pcs I've seen over the years have been multiclassed (less in 4e, more in 3e, but overall). But of course, I played in and dmed games that kind of cultivated the sort of "excessive multiclassing" that you don't care for, so it makes sense that there would be more of that kind of thing in my experience.
I'd wager that the more experienced of a roleplayer you are, the more likely you are to want to tinker with your character. In 3E, that tinkering was via feats and the simple multiclassing system.
I've got two comments about that. First, it's inaccurate for you to say I didn't care for 3E's multiclass system (excessive or not). I played more 3E than any other edition, and I don't think a single character was single-classed. I've got much love for 3E, and 3.5. But it did get crazy at the end, what with the aforementioned unbounded madness.
Second comment is a repeat: as great as 3e was (or wasn't), it was 3e--and 5e is an opportunity for a fresh take. That's what inspired my initial post: a desire for the designers to look at the past, sure, but to let that past
inform the new edition. Rather than dictate the new edition. In short: I'd prefer that 5e not be a 3.999e.
the Jester said:
However, I will say that I disagree that 3e's system was inherently bad. With improved implementation, it could work great. For instance, if all classes gain attack bonus at the same rate and some (e.g. fighters, paladins, barbarians, etc) get additional attack bonuses as class features at certain levels, then the attack bonus problem more or less solves itself. The proficiencies issue is pretty easy to address just by adding a line differentiating between "Starting in this class" and "multiclassing into this class". And so on.
I hope my comments above clarify that I don't think 3e's system was inherently bad--for 3e. With proper oversight a similar system might be able to work, and not collapse under its own weight. That said, I'd prefer to get a look at a fresh take first, that started with "why would a player
want to multiclass?" and grew from there.
the Jester said:
I'm with you on having multiclassing be optional. However, I don't like being forced to include feats to use multiclassing options. A 1e-style game might have multiclassing with no feats, for instance.
Fair enough. How about new classes that are a thoughtful interpretation of classic multiclass combinations? Ah yes, bloat.

I think you dislike bloat as much as I dislike 3e-style multiclassing in 5e.
Sure, but I'd rather avoid race and class bloat. I'm all for a book of spells, a book of feats, even a whole book of backgrounds. I'm all for monster book after monster book, a 5e Frostfell and Sandstorm and Stormwrack, books on the planes, setting books, etc. Extra base classes and races, though? My least favorite stuff.
Me, too. But you acknowledge base classes are coming. So since they are coming, wouldn't you prefer a Wizard/Cleric to a, I don't know,
one of the fringe classes from 3e's list of 175 base classes? That's not a knock on 3e; it's a recognition that class bloat will happen, so might as well be make-sense classes like common multiclass archetypes.
the Jester said:
While a fighter 2/ranger 2/rogue 3/monk 2/bard 1 is "not D&D" to you, it certainly is to the guy whose favorite character is that fighter 2/ranger 2/rogue 3/monk 2/bard 1. And what if he's not built that way as a power build (frankly, it doesn't look like one to me), but rather just through the natural evolution of his character (fighter first; wilderness time to gain ranger; urban adventures teach rogue; retreat to the wilderness and learn monk skills; then travel and tell your tales, bard)?
I like 3e multiclassing precisely because it allows finer tuning that previous versions of the game's class systems have.
Again: me too. I loved 3e's multiclassing when I played 3e, up until it got too crazy. My initial post linked to the
forbidden fruit I discovered at the end of that dark trail. And that sort of character that skips around to five entirely new professions every 1-2 months of in-game time, and gains instant mastery over a suite of base-level abilities in those new professions? The wizard who, literally
one day, wakes up suddenly knowing how to wield
every weapon with the skill of a career soldier? 3E's multiclass system serves that character well.
So well, that I don't think it needs to be done again. That system exists; 5e is an opportunity to try something different. To meet the needs of that player in a new way. In 3e, if you wanted your fighter to pick up a little magic you had to devote an entire character level to the endeavor, with the associated crappy attack bonus and low hp. In 5e, the Disciple feat gives you cantrips--you can do a little magic, while still remaining a fighter. In previous incarnations of the 5e playtest, that minor ability met the pre-req for gaining a familiar (prereq: ability to cast a spell). Now you're a fighter who can light his own cook fire, amuse his pals and confound his foes with illusions, kill pests with a pointed finger and whispered word, and command a magical companion. All while remaining as puissant a fighter as ever. How cool is
that?
That example hits at what I was talking about originally: identifying the player's desires. In your example, see if this satisfies the hypothetical player: fighter class. Takes a background of "warden" to give tracking ability and wilderness lore. Spends a feat (hiss! feats! but bear with me) to gain sneak attack and a 1d6 expertise die to one specific type of thievery (pick pocket, or locks, or traps, or flim-flam). Spends another feat to learn unarmed attack. Spends another to gain one use of bard song. You've dabbled, but you're still a fully-effective fighter. And each of those dabblings gives less than a full level's worth of benefits of the dabbled-in class.
Perhaps a hobby/interest system that rides shotgun to the level system, representing what your character pursues when they're not pursuing their main profession. For true seismic shifts, where the character puts down the sword in favor of devoting themselves fully to magic? I suppose 3e is pretty good at representing that specific case.
Or something entirely new? If the player is that uncommitted to a particular character, I as dm would suggest having that player roll several characters and play them all, at once.
Point being: even the player of the fighter 2/ranger 2/rogue 3/monk 2/bard 1 must be aware that he's building a custom class, that D&D does not offer custom classes (that's why he's forced to build it himself by taking multiclassing to a crazy extreme), and therefore his fighter 2/ranger 2/rogue 3/monk 2/bard 1 is "not D&D."
the Jester said:
Firstly, I think it's silly to ignore powergamers when designing a given major subsystem of the game, just as it would be silly to ignore the heavy-roleplay crowd when designing that same subsystem (though, probably, certain of the game's systems- combat, interaction, etc- have to veer one way or the other). Why arbitrarily exclude them from the fun here? Multiclassing is a big enough part of a ton of campaigns; it should serve everyone.
It's not arbitrary. It's deliberate. But more importantly: I'm not suggesting to
exclude powergamers from fun, or to have a system that doesn't not serve everyone. I believe I advocated
ignoring powergamers when coming up with a new solution to the player motivation behind what has, in the past, been serviced by multiclassing. To take a step back and think about why
other players would want to multiclass. We already know why powergamers like to level-dip. So ignore that, for now, and think about the motivations of the other
player types.
I keep linking that because it's useful shorthand for D&D's customer base. Everyone's a mix, sure, but I think it's a useful exercise to consider the other player types. It might lead to an innovation.
the Jester said:
As far as new ways of doing multiclassing, while I'm open to the idea- I like the idea of "dip" feats, for instance- if we're going to use a new system for it, it should be an improvement over the 3e system. We tried multiclassing via feats in 4e; frankly, while it's not quite underpowered, it more or less sucks. Something new, different and better? Sure. I just haven't been sold on anything else yet. I think 3e had D&D's best multiclassing, though, so I'm obviously prejudiced.
I'm excited about the possibility of something new. 4e feat-based multiclassing did suck, but it was necessarily tied into a lot of 4e's other systems. Totally agree that 3e had D&D's best multiclassing so far. So you can see why I'm enthused to see 5e, which is most-like 3e in terms of base mechanics (think about it--it's not 1e or 2e's soup of systems, and it's not 4e's cacophony of powers), try 4e's feat-based system. Yuck, what an awful sentence. Anyway, I'm hoping for a best-of-both-worlds scenario. Or something wholly new.
the Jester said:
Fair enough, I guess I did get carried away there, didn't I!
I think the solution is better implementation. The main problems with 3e multiclassing can all be fixed pretty easily IMHO. They just need a little attention and- dare I say it?-
playtesting!
No worries.

I certainly think it's possible to fix 3e multiclassing (though with more than a
little work). And
yes: more playtesting needed.