That's a hard question to answer, because whilst there are surface things that remain the same (for example, the existence of the Feywild, Shadowfell and Elemental Chaos, gnolls being creations of Yeenoghu), under the surface, they're a lot different. I can't speak of any changes to the Feywild or Shadowfell off the top of my head, but the Elemental Chaos is now more of a third "intermediary" plane - like the Astral or Ethereal Planes back in the classic Great Wheel, it's basically a barrier between the Elemental Planes and the land beyond, plus with lore tweaked so that the Elemental Planes as a whole functions more akin to the "Pillars of Creation" in Exalted. Likewise, though gnolls in 5e share the same base roots as 4e's gnolls, they have none of the versatility in characterization that 4e's "Playing Gnolls" presented us with.
I will thank you for pointing out the Lamia switch-over, though, because that's one of the few things I prefer about 5e to 4e. I mean, the skin-wearing carnivorous beetle-swarms of 4e were an awesomely creepy monster, but the weirdly sexy beast-taurette monster is more what I associate with Lamias. I just wish they'd gone a step above and either brought back the Lamia Noble or else made all Lamias snake-taurs, like they are in every non-D&D presentation of them.
