Class Power Source

[Kurald Galain]

I suspect WotC limited them mostly because because many players only seem to be able to handle a limited number of options.

Pretty much; WOTC got player feedback that certain players enjoy having fewer options. This kind of player would likely play a Fighter or Rogue in earlier editions, so the 4.5 Fighter and Rogue were simplified to appeal to this kind of player. I think that didn't really work out, but that was the intent.

Knight and Slayer aren't terrible or anything, but monotonous.

Yes, that. Well, Knight ended up pretty bad; Slayer and Thief are solid middle-of-the-road options.

Battlemind and Ardent also seem to want to spam the same at-will*, and their dailies tend to tack on another Augment or otherwise have a similar stancelike "gain XYZ for this encounter"

Whereas Slayer has only few options, B'mind and Ardent actually have a lot of options but only a few of those are worth using. I'd say that's a design flaw. Also, B'mind and Ardent really suffer from a case of "WHY does this even exist"; both from a flavor perspective (you have to do quite a lot of mental leaps to find any famous fictional character that clearly represents this class and not another one) and from a mechanical perspective (they've basically "an existing class BUT with power points", and turns out that's not that big of a distinction).