They replaced automatic brightness with adaptive brightness, the difference being that you specify a range in the slider and stays within that range. If you slide it all the way to the top, it'll hover around high brightness. At least I think that's how it works. Whereas automatic brightness in KitKat would utilize the whole spectrum of brightnesses.
Yeah, adaptive brightness is basically a "semi-auto brightness." You set a brightness level and then the system uses the ambient light sensor to keep the display brightness consistent relative to the ambient light. In theory at least.
For the OP, if you disable ambient brightness you will likely see that the display gets brighter when set to the max level.