Only a graphic upgrade of a old game
Difficulty:
I played serious level, it is so hard but in normal it is easy. It added the co-op mode made player had unlimited respawn and you could compete with other online players for a news high score.
Gameplay
Unfortunatly some map was not 100% remake, such as Valley of the King (level 4), the slippy floor was removed (http://www.youtube.com/watch?v=u_7Z2_mhpmQ at the start of this tube) But it still fun to unlock all secrets.
Overall if this game released in 2005 for sure it is very popular, but it just a engiee test or rasied fund for the release of Serious Sam 3, but if you want to buy it, just keep checking Steam daily discount. Also if you own both game (or just Second Encounter?) you can download a fusion pack, more fun!