The 'queer thing' that the Time Traveller notices about the Eloi is that they lose interest in things very quickly. Although the Time Traveller is a complete stranger, so unexpectedly arrived in their world, they don't bother about him for very long. He finds this frustrating as he would really like to communicate with them properly, but he realises that they are not capable of any sustained concentration or effort at all. This is because their intellect has decayed; they have regressed to the level of five-year olds.
The Time Traveller is saddened to see the state that humanity has been reduced to. As that day draws to a close he steps out to surveys the scene:
The calm of evening was upon the world as I emerged from the great hall, and the scene was lit by the warm glow of the setting sun. (chapter 4)
This picture of the setting sun could be taken as a metaphor for what has happened to humanity: it has passed its peak and now is in slow, steady, quiet decline. The Eloi are carefree, pretty and charming but they have almost wholly lost their mental capacity.