Finally bit the bullet and played through this (long after doing the freeware version ages ago).
Since Xen has been discussed to death (8 years for that?), I have three big things that bothered me about this mod.
- Vortigaunt charge attacks are now dodgeable. Completely ruins the point of the enemy when you realize this. They were designed so you have to either hide when you hear them charge up, or take them out quickly in a specific order if hiding wasn't feasible. Now you can just strafe and be safe. Huge miss from the mod team. Also they gave them the Houndeye AoE attack too for some reason.
- They attempted to bring Half-Life 2 combat over to Half-Life (for the worse). The MP-5 is the big loser here as each clip now holds roughly half the ammo (30 vs. 50 rounds) and you're limited to three grenades. In the original, the MP-5 was a relatively weak yet very versatile weapon that traded the preciseness and a little damage from the glock for a big improvement in firing rate. Now it's almost not even worth using with how often it needs to be reloaded. It may make more sense in reality (MP-5 uses 30 round clips, although 50 round drums do exist), but it makes combat far less interesting as you'll often have to reload multiple times to take out a HECU grunt.
- The land mines in Surface Tension are now visible. Kinda removes the entire latter half of the chapter title.
There's plenty to like and the general aesthetic is pretty faithful (if you don't mind the darkness), but a lot of the design decisions are clearly for the worse. Ended up replaying the original afterwards and it's easily the superior product.