The firsts expansions weren't that great IMO. I mean I played through the first one but just the demo for the second one made me vomit. The voice acting alone was horrid.
2 is a lot less repetitive corridor crawling and everything is so much more interesting...and doesn't look like its made of wax 
I'm a little weirded out that 3 has been handed off to another studio, what the hell is Monolith doing? I know they're still working with Day 1 and all that but that is just bloody weird.