This game is great. It takes most of the fun elements of the original while stripping out some of the minor annoyances and bits that didn't age well. I LOVE half life 1, and even tried speedrunning it for a while, so believe me when I say almost every change is for the better.
- Xen is amazing, totally redoes the original and turns it from a strange and unintuitive slog to an action-packed exploration of a new world.
- They simplified some of the more annoying levels and contextualized some previously nonsensical architecture
- The world-building and character they achieved is amazing.
- The graphics look stunning, it's hard to believe this was made in the Source engine.
- They merged the several tiny levels of half life into much bigger levels. Level load times are worse than the original (which is unavoidable) but you don't have to load nearly as often so I think overall it's a good trade-off.
- They removed the ability to tou-jump. I don't know why, maybe it had something to do with sequence breaking in Xen, but it's something I enjoyed and I'm sad it's not here.
- There's a couple of weird difficulty spikes, but only a couple. I can think of two off the top of my head: the combat sequence at the end of Questionable Ethics and the Xen portal.