Got halfway into "We've Got Hostiles", here are my thoughts:
Graphics: Pretty decent considering its an 8 year old engine. Source's water is still damn pretty to look at, but models haven't aged that well.
Sound: They added a lot of music of the kind that was frequent in Half Life 2. While those electronic vibes are nice, they seem a bit out of place here. For example, once you put on the HEV suit, this foreboding track starts playing, which doesn't really fit with the tone of the game. It's a lot less claustrophobic and lonely with the music.
Design: I last played HL1 about five years ago, so my memory may be a bit sketchy, but it feels like they've added a lot of their own stuff into the game. Some of these things are nice, like defining characters from HL2 like Barney, Eli and Kleiner, while others seem a bit over the top. For example, there's a lot more scenes of scientists being murdered by the military than there were in the original. Also, the military almost immediately start to identify you as Freeman and make you somewhat of a priority, when in fact it wasn't until much later when they caught on to your actions. Overall these things dampen the horror aspect of the game. Whereas previously you felt completely alone and vulnerable in a huge facility, now you run into NPCs pretty frequently, although this may change in the later chapters.
Gameplay: It's pretty much like in the original. One change I've seen so far, are flares, which can be used to light enemies on fire. Nothing worth noticing really.
Overall I'd say it's a very good conversion, although maybe they went a tiny bit overboard with modernising it.