The idea behind 20 level base classes is that the game is 20 levels long, and people should not be forced to multiclass if they don't want to. Ideally, taking only monk levels would be just as viable as a Wizard 3/Master Specialist 2/Incantatrix 10/Archmage 5, and would be a simpler option for newbies or for people who just don't want to make a character that complicated.
By making a base class that is less than 20 levels, you are forcing people to engage in a certain part of the system that they didn't have to before.
Also, it offends some people's sensibilities when you say "Ooh, sorry, you can't continue taking levels in that class you identified yourself as since we started playing. Try one of these instead."
I don't think multiclassing is actually any more complicated than single-classing, especially if you don't have to choose your classes yourself if you don't want to. Take for example the much-discussed Paladin. It is basically a pre-built multiclass: you start off all martial with extra damage and immunities and save bonuses (among other evil-hunting abilities), then you gain spellcasting which is a whole different thing. Heck, the Fighter might be the most complicated class of all to build for, with having to choose your own features and keep track of prerequisites for them and try to get the most synergy for a style and action sequence.
I think that taking a single non-spellcaster class to 20 leads to this problem: the spellcasters become inherently both more interesting and more complicated. The thing is that even a single-classed caster does not have a single defining concept or feature but rather several. The spell selection is akin to a variety of class features. For example, a specialist necromancer isn't
just a necromancer, he is also a decent diviner and transmuter and whatever other magic he chooses to learn. Even if he only chooses Necromancy spells, there is still enough variety between them to cover more than one thing. Multiple ideas, multiple concepts. A multiclass in one. The trouble with RPG game balance is that it isn't just a matter of measurable power. Perceived imbalance is just as much of a factor in making the game less enjoyable, perhaps even more. When one character is multi-faceted and another character is only one, there will be a perceived imbalance even if the one-track character is powerful.
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There was an essay I read a while ago, written by a guy who works in video game development. I've forgotten the site and his name and even the first-person shooter he worked on, but the idea he was talking about really hit me. He was talking about how the players of the game were sending a lot of complaints about the reload times of the guns, saying it was too slow. The developers took all feedback seriously especially when it is shared by a large portion of their player base, and so went about solving the problem. The thing was that the reload times were already really really fast. Not only did making it faster cut into the realism (an important part of that game), but after testing a faster reload animation they found that people were still complaining about the reload time. So they thought about it, and realised that the problem wasn't that the reload time was too slow, but that it was too
boring. They redrew the animation to make the action more exaggerated and cool-looking and the complaints about slow reloads went away, even though the new animation actually took more time than the old one.
The lesson he took from this, and which was the topic of the essay, is that often what gamers think they want is not the same as what they will actually enjoy and that a designer has to both read between the lines and read behind them to find the best solution.
This goes for RPG design also, and I think it can be applied here. I'll take as a comparison another RPG: Warhammer Fantasy Role Play. In WFRP, multiclassing is an expected part of progression. Once you finish your first basic career (which happens at about 1000 xp), you then become something else. There were some people who complained about the "forced multiclassing", right from the first edition and into the third, but it remained across the editions. Why? Because it works! Yes, you cannot be a soldier or a peasant all the way through the campaign, but the thing is that as the game progresses and the characters become more heroic and important, being just one thing is no longer thematically appropriate. I think a similar thing applies to D&D. At level 20, you are no longer merely a fighter. It doesn't fit with the scope of what you are doing.
Many base classes change as you go on, whether it is the monk gaining supernatural powers or the paladin and ranger gaining spells. Separate them into their individual concepts (Martial Artist and Enlightened One, Slayer of Evil and Living Saint, Huntsman and Woodland Caster), and you open up new combinations between the sectioned classes while still making it possible to build something like the original 20-level class. As for classes that cover a single idea, they shouldn't be like the original 20-level class because at 20th level having a single thing to identify the character as will make it no longer fit in with the game.
In short: single-class is no guarantee of an easy-to-build and uncomplicated character, and at high levels identifying the character as a single thing that it was from the start may well be a bug rather than a feature.
Just throwing this out there, since the "pro 20 level classes" side is kind of under-supported right now.
T'is fine, I love debating these things. Even if nobody agrees with anybody at the end of it, the exchange of ideas is healthy for everyone involved.
EDIT: moved over from the old thread. If you're wondering where all this came from, it was over here:
http://www.minmaxboards.com/index.php?topic=8393.0Sub-discussions ahoy!