The HD version introduces new enemy types, (barely) better graphics, more achievements which are way more fun and intresting, an updated engine, a new playable character (The Doc) and workshop support. Theres also a sandbox gamemode which I didnt play because Im just not a fan of that type of stuff.

Otherwise its sadly the same game and costs money instead of the original. Its more fun because of the new enemy types but unfortnuately you may spend 1-2 hours longer on that game than the original before it gets boring again. Its not a bad game at all and I like the improvements. I just dont think they make me wanna play it that much longer.