You can see a full list of all of the changes here, in this article I will talk about some of the changes which impress me the most, however check out the link above for the full breakdown.
Firstly I will talk about gameplay changes, now Hardline Pro will only be able to re-roll Care Packages, prior to this Hardline Pro users could re-roll Sentry Guns and SAM Turrets, we spoke about this here.
According to Treyarch it seems as if spawning should be greatly improved, I must admit I have not noticed much difference myself (especially on Nuketown), however I have only played Black Ops for a couple of hours since the update.
If you are a Ninja Pro user you should notice some sound mix improvements, if you are a Sniper you should see slightly improved accuracy, increased headshot multipliers and also if you equip your sniper with a silence you headshots will always be one-shot kills.
Two other gameplay changes should be extremely popular, the red diamond player indicators which show up when using the Chopper Gunner, Valkyrie Rocket and Gunship will take longer to display when a player spawns, which should mean that players get killed less by these killstreak rewards, also if you hate getting stabbed you will be glad to hear that knife lunge range has been slightly reduced.
The rest of the changes either relate to connectivity or rarely occurring issues, such as spawning outside a map, Valkyrie rocket exploding upon release and people being able to keep their killstreaks and classes after prestiging.
As previously stated it is worth checking out the official Black Ops forum post for a full breakdown, which change are you most happy with?