From Alien Swarm: Reactive Drop Steam page description:
Alien Swarm: Reactive Drop extends Alien Swarm, bringing more of
everything: maps, aliens, game modes, guns… And most importantly
Steam Workshop support.
- Tactical co-op for up to 8 players with a top-down perspective
- Steam Workshop support for community maps and challenges
- New co-operative campaigns
- Challenges: Modifications of the game, just like Mutations in Left 4 Dead 2
- PvP: Deathmatch, Gun Game, Instagib, and Team Deathmatch
- Singleplayer: Play with improved bots on all our official maps
- New Aliens: HL2 antlion guards and more
- New Weapons: Desert Eagle, Devastator, and Combat Rifle, with more to come
- Over 100 Steam achievements
- Leaderboards: compete with your friends for the fastest mission completion
- Improved spectating: see hacking minigames and mouse movements in real time
Are the games mechanically identical aside from added content (maps, items and enemies)?
After playing for a while, the tutorial level seems similar, if not identical and graphics appear to be same, including enemy death animations. All seven levels from the original can be selected in the main menu.
Best Answer
Having spent over 100 hours in both games, the Steam store list page is pretty spot-on in regards to improvements/additions. The only mechanically different thing that I can think of is that several map exploits were patched, which only improves the game further.
All character traits, weapon balance, dialogue, movement, and difficulty were all preserved. Granted, Reactive Drop lets you crank up the difficulty by several magnitudes higher than Brutal now, but the existing difficulties are all the same.