It wasn't that long ago where Treyarch was the "bad" developer of Call of Duty titles. CoD 3 and World at War are now distant memories as Treyarch has hit something special with the Black Ops series. The first title has grown on me over the years and is now my favorite of the CoD titles, eclipsing the first Modern Warfare title. Treyarch has continued their usual technical prowess (they do great fires and explosions) and improving the storytelling appreciably. Better engine development would be appreciated --- and will be needed before their next title --- but it doesn't detract here.
You don't just swap characters but time frames. You are fighting in the 1980's and 2025. But you aren't the cog of the story. Your core enemy is the focus of the title. Everything you do is in response to his plans. Raul Menendez is one of the better villains in recent gaming history. From drug lord to cult leader, he engages in a wide array of unpleasant actions. And you have to serve as the men who chased him in the 1980's --- and who directly led him to become the cult leader he is in the future. He's not pure evil (no, he's not nice, but they DO explain why he does what he does and a compelling villain has to believe what he/she is doing is right).
They do tie it in the original as you, in the 1980's, play Alex Mason --- the brainwashed protagonist from the original. The game makes it clear that you are not over what happened to you, but it's also clear that your teammates are not aware that you're not over it. In 2025, you are Alex's son David, who has to deal with the ramifications of what Alex and his allies did in the 1980's. It is coming close to giving you concrete consequences for what you do. If developers start pursuing this line of gaming, this could be an excellent change and an opportunity to finally inject a bit of life into the FPS genre.
The game is, of course, linear as all royal hell, but I sincerely enjoyed it regardless. I personally preferred the loadout of the future warfare and the FPS play...but did not much enjoy the "Strategy" missions that work with the storyline. The game is difficult, not as cheap as in prior titles, but difficult regardless.
Visually, the game has gotten some knocks and I'm not getting it. I actually liked the way it looked. It did look really good. I didn't notice all of the hiccups people are claiming. The models look solid. The environments look solid. The animation is solid. I think it is a really excellent visual title. I just do not see any real problem. Your mileage, apparently, may vary but I like it a lot.
There isn't a ton to say about it. It is a great game. It is well worth playing. Everybody should give it a try. It is likely the apex of this generation of FPS and I'm looking forward --- not the next FPS titles, but the ones the year after. It could be something special.