I think one of the best things about AVP 99's aliens, which hasn't really been seen in any aliens game since, was their nasty habit of craking if exposed to heavy gun fire. Yeah, in AVP 2010 they burst if you torch them, but in AVP 99 they could explode in a shower of acid just from an intense pulse rifle burst. It made contending with them a real threat to your health.
I get why it's been tone down (for mass market appeal) but it really was the best depiction of the lethality of acid blood.