Some variations on a theme. The Second Encounter should be regarded less as Serious Sam 2 and more an expansion pack; befitting The First Encounter's existence as a throwback FPS, you could view Second Encounter along the same lines as "Thy Flesh Consumed" for DOOM or Mysteries of the Sith for Jedi Knight. On the surface, part of this is the varied settings; instead of First Encounter's Ancient Egypt setting, Second Encounter has Sam running around in Mesoamerica, Mesopotamia, and Mesaíonas Poland. Level design is a bit more restrained, though this doesn't say much; mostly it just means that the game can have sequences, like that castle ascent in Poland, where the player(s) need not worry about the game spawning Werebulls from outta nowhere to fling them over the edge.

I probably put Second Encounter about on-par with First Encounter. Having the change of scenery is nice (though, Mesopotamia doesn't feel that different from the first game's ancient Egyptian desert cities), but there's a bit less flow due to the Second Encounter's segmented campaign. Otherwise, my opinions on the First Encounter stay pretty consistent with this follow-up. Serious Sam HD bundles First and Second Encounter together anyway, so pretty natural to treat this as a Sonic 3 & Knuckles situation.