I don't know if it's related to what you're hearing as metallic, cold, harshness, etc., but possibly.
Mike did a great job of explaining the essentials: in a nutshell, when a digital audio signal inside your computer hits your audio hardware and is converted to the analog domain, there's often the existence of audio outside of what the hardware can handle, so it attempts to move those super-high frequencies *into* the audible range, and the result is aliasing. It's typically undesirable because it has been artificially placed there, and therefore changes the sound to some degree, and, yes, it could be metallic, or cold, or harsh, or a number of things. Anti-aliasing attempts to remove that aliasing by removing (or ignoring, or redistributing) the frequencies above and below the sonic range before it gets converted to analog by your audio hardware. And it works, but it also burns a lot of your CPU, which is why developers often give you the option of picking 2x, 4x, 8x, etc. oversampling. I almost always prefer the higher magnitude, but not always, and with some plugins it's just too much math for too little sonic improvement.
I'm not too familiar with the Theta technology but it sounds like they're applying anti-aliasing in a way which they feel merits advertising. I'd be curious to hear it. Or, rather, *not* hear it.
My guess is that it's not going to be different than what we're accustomed to with other methods, like S-Gear's, which I hear as natural.