The most notable change is probably the Marines. Not only did our lead programmer rewrite their AI entirely, their values and behaviour were adjusted a lot too. They're no longer aimbots, but are instead much more like HL1 in that they run around a lot (and move fast), and are very good at finding angles and applying pressure on you with nades and tactics. Testers and the team seem to think they're way more fun and less frustrating to fight now, and I certainly think that too. In particular some of the tougher set pieces like the QE ambush are really fun now because there's a lot of unpredictable, run and gun action. The LAV is also tough now.