- Half Life has much faster run and gun gameplay (stemming from it's quake roots no doubt), while in Black Mesa it's much slower paced
- The AI in the game is completely different, Black Mesa's soldiers feel way more aggressive, again encouraging a slower pace of play compared to the original
- Lots of levels have small changes - some are cool, others kinda feel like they're just different for the sake of being different
- Xen itself is completely different; Unpopular opinion, I liked the original! It feels otherwordly and alien and oppressive, the new one is certainly pretty but lacks that atmosphere, imo
Black Mesa is a great game, one thing I have nothing but praise for is it's presentation - it's really nice to look at and they did a bang up job with the graphics and animations. But which is better is a matter of opinion, and personally I much prefer how HL1 actually feels to play.