Treyarch didn't screw up, coming from someone who hated CoD3.
The feel of the game is much darker than CoD4, and quite a bit like the dark atmosphere of three. I think it fit with 5 much better than 3, but still wasn't quite as good as the other CoD games which, despite the games content, weren't that dark or forboding. Treyarch seems to want to turn the games grittier and darker, when that really hasn't been what Infinity Ward has done with the series. 5 does a much better job with the dark, forboding feel than 3, which really forced it to be dark in my opinion and really went off the deep end.
The Immortal companion characters are far less enjoyable than previous CoD games, save 3. There is no feeling of attachment to Roebuck, and Polonsky never does anything memorable in the American campaign. Reznov from the Russian side is enjoyable, but very over the top. Chernov, the other Russian Immortal is not likeable, and really pitiable as he gets pushed around by Reznov. There are no great characters in the game such a Captain Price Foley.
5 game is the easiest CoD I've seen. After playing through once on Hardened I'm buzzing through the game on Veteran without an issue. It took me a couple weeks to finally finish 2 on Veteran, and there are still 2 missions on 4 I haven't beaten on Veteran.
Graphics are probably the only thing 5 has over 4. The addition of dismemberment alongside beautiful flame effects really step up the graphics, and in my opinion allow the darker atmosphere to work. Some character movements can look awkward, but I've never seen a CoD that didn't have some strange NPC movements.
Sound and voice acting are both very well done, as with 4. Some problems arise with incredibly cheesy dialogue. Lines like "As long as you live, Dmitri, the heart of this army cannot be broken" made me want to roll my eyes.
The weapons are fun and satisfying. They sound great and the variety is the best I've seen in a CoD WWII game.
Multiplayer is excellent. More or less a carbon copy of 4's set in WWII with the addition of vehicles which I don't like. I wish they hadn't included tanks, but luckily vehicles are not overly powerful. One guy on foot with AT gear versus a tank can be a barely vehicle-favored fight if the person on foot knows what they're doing. Get two guys with AT equipment and the tank can be in trouble.
I think the biggest thing I'd say about this game is that it's not CoD3 take two, and that's a good thing. Treyarch didn't suck it up like it did before. I'd highly recommend this, even if you have massive apprehension due to CoD3.
The game is great, not amazing. Unfortunately for CoD5 the bar for CoD games in the past has been amazing.