We have sailed 3 halloween themed cruises, even before they were sold as such. I found that with each cruise, some things changed, some stayed the same.
Dressing up - trust that a LOT Of people dress up for the evening they choose for their halloween party. And I've seen some really cool stuff so that was a lot of fun (with contests).
Our first sailing (Dream), they had huge amounts of candy EVERYWHERE, especially at the entrance to buffet, dinners. They have a dance party which is fun with the halloween music and the gang in their halloween costumes. The kids got a trick or treat bag in their room to hold candy.
Now our last cruise which was 7 nighter on Fantasy, they had changed things up and had a Halloween tree with a storyteller which was really cool. Now on this cruise, they didn't have the large amounts of candy sitting around but had some sort of cheesy games for the kids and they got candy upon completion. They had some folks around certain areas where the kids could go up and get candy. Again, trick or treat bags were provided in the stateroom. Cabana's has carved pumpkins all over and they are well done.
So keep in mind, you'll have 2 dress up nights on these cruises. 1-Halloween and 2 - Pirates Night.
We really loved it personally. But as I said, we noticed out of the 3 we cruised, there was always enough different from our prior sailing that it was nice yet funny how you wish some things stayed the same. ;O)
Again, I got just as much enjoyment looking at everyone's costumes as I did anything else. It's amazing how much work some folks put into their costumes. WOW.
As far as decorations, yes...the decorations will be around the entire cruise but there is only one day of the cruise dedicated to the halloween festivities. So I think if anyone is concerned about being romantic, I think any situation can be as romantic as you choose to make it. Heck, you could dress up as a groom and bride! ;O) Enjoy a second wedding if you will.........
Also keep in mind, if folks are at the holiday celebrations, you may have more room/space to yourselves in the adults only section. ;O) If you liked Carnival, Disney may just blow you away. Honestly, there's NO comparison. The beds are awfully comfortable! Whatever your decision, have fun and enjoy. Don't let it set you back.