I am going to try and dispel some of the things I believe are either myths or not directly related to if a system is class based or not.
Many of the examples are going to use D20 and GURPS because I believe many gamers are familiar with these systems. It doesn't hurt that I am very familiar with both of these systems.
Myth: A game system has to be balanced
I don't consider it necessary that a game system be balanced. It is almost always important that the game be
fair, but that is different than balanced. There are also games, like "The Great Dalmuti", which are intentionally unfair yet still enjoyable.
I believe that it is much more important that a game be fair to all the players than that it be balanced. As seen by
this statement by the GURPS line editor, not all games even try to be balanced.
Q: What the heck is a "point", anyway?
A general quantum of [Usefulness (averaged over all situations) x Rarity (averaged over all settings) x Difficulty (esp. for skills)]; i.e., basically, a highly-abstract and nearly-meaningless place holder for those who insist upon detailed accounting. In a campaign with more-or-less normal characters of similar background and abilities, points have some value as a tool for comparison, but by and large, the concept is almost intrinsically meaningless in a truly "generic" game.
There are classless systems that do attempt to use points as a balance mechanism, the best known being Hero/Champions. One of the design principles in that system is that the defense costs half as much as the attack form it protects against. That doesn't mean that all point systems do things that way.
The points in GURPS are more a measure of how unusual a character is than how powerful it is. Since every character would be choosing skills, advantages and disadvantages from the same available pool of capabilities, this can still be fair. It just doesn't pretend that a high skill in Celestrial Navigation is automatically more useful than a high skill in Desert Survival.
Game balance is heavily dependent on the type of campaign you run. As anyone who has read the D20 Rules forum can attest to, not everyone agrees what is balanced. Even within this thread, not everyone appears to agree how useful within D20 the skill set and social background of a Noble would be to a Wizard. Some have said they would just modify the Wizard skill list, while others would penalize the Wizard's magical powers by forcing the character to multiclass.
Given that not all abilities will be equally useful in all campaigns, it is understandable that some game designers would not put a high priority on this.
Myth: I can't balance an encounter without a class system
This is a
very rough paraphrase of what some people have said in this thread. Calling it a myth is a bit of an overstatement, so please forgive me for that.
Balancing an encounter is more dependent on the growth curve of character advancement than if the system uses classes or not. In a system like D20, the growth is almost exponential with sudden jumps (as you gain certain levels) in power. With something like GURPS, the growth is much smoother and more linear.
For D20, most people would not consider it fair to require that a new character come in at 1st level when most of the group is at 5th to 7th level. The lower level character would have a very hard time surviving, and quite often would not have any powers of significant use to the group.
For GURPS, I've played in games where new characters came in as 100 point characters when there were other characters with more than 200 points. The characters could still contribute to the group, partially because most characters don't increase in hit points as fast and partially because there was some useful skill or capability that was unique to their character and useful to the group. Many of them were actually better in combat than one of the existing characters with other 200 points, because of the difference in character focus.
There is also a difference in the way that encounters are set up in the two systems that make a difference. A GURPS campaign almost always uses a goal based system for awarding experience. There is no standard XP method based on creatures defeated. Instead of setting up a series of encounters, more GURPS GMs set up situations and enemy capabilities and then allow the players to choose what they can handle and how they will do so.
Since the players can choose their capabilities, they learn what skills and advantages they find useful. They will not tend to put many points in the stuff that doesn't work, focussing more on skills or advantages that allow them to accomplish their goals.
Myth: A class system is just as flexible as a class less system
In my opinion, this is a myth. I recognize this is a matter of opinion though.
In general, a class based system needs new classes each time you change the campaign background. The character classes in a science fiction campaign with FTL travel and multiple sentient species will be different than that of a campaign set in a pre-gunpowder world in which magical abilities exist. If each class has unique capabilities, you have to look at how useful those capabilities are within a specific setting. When you change the campaign settings, you now have to look at how that affects all of the existing classes and what (if any) new classes are required.
People who say the can easily customize the system are missing the point. In a classless system, you don't have to adjust the (non-existent) classes. You decide what rules you are using and everyone makes their characters using those resources.
In the example I gave previously of a Noble Wizard, it would be relatively easy and consistent for a GM to make the decision in GURPS. First the player would have to check with the GM to see how such a character could fit within his campaign. Assuming that the GM is willing and explains the campaign social structure, the player would already have all the rules needed to create the character.
How many GMs would really object to a player that wants to customize a character so it fits within the campaign background?
Myth: One system is the best in all possible genres.
Once again, this is more a matter of opinion. In my opinion, different systems (because of their differing design objectives and assumptions) work best in different situations.
A number of people have already mentioned how they thought GURPS makes the best system for Science Fiction settings, especially something like Traveller. I would say that it is a good fit for any setting that trys to be more realistic. In my opinion, it does not work well for very cinematic settings such as four-color Supers.
The D20 system has a number of feats that allow characters to preform extremely cinematic physical stunts. It also is not overly concerned with anachronisms. If you want the type of physical stunts like the TV shows Hercules or Xenia featured, this system works pretty well.
If you want to emulate the various HK style action films, you should really look at
Feng Shui.
In my opinion, people should find a game system they like and that does a good job of representing the type of setting they want to play in.