Hmm, here was my take and much is based on the fact I have extensive knowledge of both comic books, Dungeons and Dragons and a variety of spinoff games in various genres (electroncic versions, rifts, gurps, card games, etc).
Gadeteer: Batman or a steampunk themed character such as from Arcanum (pr whatever te PC game was that helped introduce the concept). Currently, the class seems a bit of a mix of both. You have some combat abiilty like Batman or Robin or Nightwing, MoonNight, etc, but also have a lot of advanced technological opportunities bordering along the lines of um, well, positron computers are like Mr. Fantastic stuff almost. Sort of. Kind of. Oh, Shi'ar computers. Like Cerebro.

There we go. So more like something Beast or Forge could make.
Elementalist: Obviously someone who used the elements. I instantly thought of Storm, Human Torch, Sunfire, IceMan, Dr. Strange and various elementals from Dungeons and Dragons as well as typical wizards and the like. Druidic magics were also in mind but as this is a superhero game, I try not to let my mind expect typical fantasy stuff. Anyway it seemed obvious the character was designed to sling spells and classic element energy.
Naturalist: Druid, particularly the prestige class called Beastmaster. I tried thinking out of the fantasy-type shell and Beastboy from The Teen Titans (a DC character) came to mind. Overall the idea of someone changing shape into animals or who used animalistic focused abilities limited the class ridiculously in my mind and murdered my interest in it. I like beastboy, I like beastmasters, but the class seems -extremely- niched into that and only that category while not really using names and terms that cover 'naturalist' very well. It also isn't a superheroish themed named- leaning heavily towards the mystical fantasy genre. I shied away from it but understood it was a brawler.
Psion: Totally self-explanatory. I knew it was going to involve mental abilities but most psychic superheroes are honestly pretty lame. It's the villains who rock since they actually use their psychic powers in ways beyond telekineses. Professor X, Jean Grey (AKA Phoenix), Psylocke (an exception to the heroes with psychic powers being lame), The White Queen, The Shadow King, Namor and Guardian are all examples of the variety of psionically endowed heroes and just how versatile those heroes can be. The white queen possessed ice mans body and used his powers in a variety of ways, Jean Grey is mind numbingly consistent at telekinetics and telepathy. Pyshic bolts and typical invisible stuff that gets old after a while. Professor X controls people mentally, makes them see illusions, even managed to make himself walk again through force of mental might (no longer an option sadly), he's wiped magnetos mind clean, etc, etc. Namor and Guardian create limited invulnerability fields around their body because their minds and perceptions are so strong. It is a form of psionic ability. Psylocke uses her mental powers to make herself invisibile, craft visible psychic blades which she attacks with, augment her physical prowess in hand to hand combat, and to distract the enemy with mental hallucinations so she can move in for the kill.
In D&D type games psionics often involves a warrior aspect or mage-like aspect. The warrior aspect being much like psylocke on Crack-augmenting strength, speed, teleportation, shrouding oneself in flame, augmenting hormones, density, growith new body parts, et-
and the mage-like part: creating astral constructs, telekinetic bolts, massive hallucinations, flight, ectoplasmic bolts, spirit summoning, mind control, levitation, etc, etc.
For the moment, Psions seem vague. In truth, all the classes seem to lack the definitive 'hero' feel, though gadgeteer is the hands down best name and skillset of all of them.
It is my hope in the future that more appropriate names (for some classes and abilities) come to light to help define the hero aspect a lot better versus the fantasy aspect. Sometimes the distinction is hard to make but the game is in its infancy so ideally it would be easier to address before it grows into the sick beast that it most surely will become.
That's all. :p
Edit: Good job on the lack of colors. - neo