I agree with this 100%.
For me, a good base class has to fit two criteria: It must be
mechanically original and it must
fill a niche no other base class can do. I'm willing to be lax somewhat on one if the other's particularly strong, though. The
society mind and
blue mage do both of these quite nicely (summary of the two: The blue mage is the Final Fantasy archetype of learning enemy spells by experiencing them firsthand, and the society mind is a psionic support class that makes the team operate as if they were parts of a single greater being), while the gemini dancer doesn't fit the niche criterion at all and gets a ton of press as if it did. A lot of it looks like mechanics for the sake of mechanics, actually -- one of the major reasons I dislike Sztany's Ultimate series, in fact.
(On an unrelated note, by the way, if it wasn't clear before,
the society mind was published in Untapped Potential, meaning several DMs who require print sources may now peruse it at their leisure. Dreamscarred hasn't forgotten about it, either, and it will be used in later books.)
Those two criteria can help you in designing other base classes. Step 1 is ALWAYS to ask yourself if your concept REALLY needs a new base class instead of just some imagination. From there, it's just a matter of mechanics.
For example...
[sblock]Untapped Potential's other base class, the Marksman, was designed as follows: We noted that there was no full-base-attack manifesting class, and when we looked at all the published full-base-attack classes, every one of them except the ranger (and even the ranger, to some extent) had a heavy, heavy melee focus. Thus, a ranged full-base-attack class would be mechanically original, and would be a good place to go. From there, we looked at places like archetypal "small-scale psionics" (i.e. Starcraft Ghosts, The Shadow) and archetypal ranged combatants (there's a very powerful ability there drawn from classic Western films, for instance), and came up with interesting ideas mechanically (such as the ability to develop a signature weapon style). Finally, we put it all together and pared it down until it looked reasonably balanced. [/sblock]
To illustrate further, here's some WotC examples.
[sblock]Duskblade: Fills a niche no other single class can (fighter/mage, a popular niche; the psychic warrior comes close but feels completely different since it can't blast), sort of mechanically original (not so much, but what mechanics are there are solid). Conclusion: Good class.
Warlock: Fills a niche no other class can (and that niche can vary somewhat depending on what you choose, but always having magic at your fingertips has a certain appeal), mechanically original (Self-evident!). On the plus side, it's dripping in unique flavor (which was enough to lure my brother, who NEVER plays anything *but* The Half Orc Barbarian, into trying something new). Conclusion: Good class.
Samurai: Not mechanically original (all of its abilities are fighter bonus feats, essentially), fills no new niche (a Lawful fighter could do this). Conclusion: Bad class.
Swashbuckler: Doesn't fill a new niche (warrior rogue, light fighter), not mechanically original at all. Conclusion: Bad class. But insightful strike's good, so it's worth considering as a dip.
Beguiler: Doesn't fill a new niche (sneaky spellcaster; illusionist comes close), but executes it with mechanical elegance and brilliance. Conclusion: Good class. You'll note that this is the reverse of the duskblade (which fills the niche but isn't mechanically original), yet both are good because their better side is as good as it is.
All of these work without introducing a new system either. If I wanted to, I could laud the Expanded Psionics Handbook, Pact Magic section of the Tome of Magic, the Totemist from Magic of Incarnum, and the Tome of Battle classes as "good" (they all fill various niches and do so with mechanical elegance) and the rest of the Tome of Magic and Magic of Incarnum as "poor" for similar reasons (MoI doesn't fill any new niche except for the totemist, Truenaming fails on so many mechanical levels, and shadow magic is weak on both fronts but shows promise).[/sblock] Are you seeing how to design a good class from this by now? I'd hope I'm being helpful.
If we're including variants of existing base classes, then my money currently lies on (in no particular order)
Seerow's Fighter*,
OneWinged4ngel's Paladin,
my own Marshal,
BlaineTog's Soulknife, and
RadicalTaoist's Ranger (even if the latter may need a bit of tweaking; it's getting intensive playtesting in our group now, right alongside Blaine's soulknife). All of these except the ranger are intended to "fix" an existing class, replacing it altogether (Seerow's fighter is the result of many, many long discussions of what the figher's lacking as a class, OW4's paladin is a reimagining from the ground up of what a PALADIN means, Blaine's soulknife fixes many weak mechanical flaws while giving the soulknife a niche of its own isntead of a bastard niche between stealth and tanking, and my marshal makes a dip class into something worth taking as a defining class). The ranger is meant as a variant that can be used in addition to normal rangers, trading mystical abilities for Tome of Battle maneuvers.
Hope that helps.
* I haven't yet read Otto's classes beyond a cursory skimming, but I like what I see there, particularly the hexblade.