I suppose this is only fair.
1. All the boats seem to look the same. You've got your main shop, casino, bars, restaurants, and deck -- you'll soon be able to find all of these places even while very drunk, no matter the boat! Further, most seem to have a 1990s Vegas vibe, right down to the weird blue casino carpet. Some of the carpet and wall designs even remind me of the awful patterns picked for airplane seats. Some parts of the boat may smell like airplane too.
2. You'll have THAT couple. You know, the one who is on their 16th cruise. They'll talk about how they know all the "tricks" to cruise-riding, and spend most of their time avoiding safety regulations. But let's be real -- there aren't many tricks of the trade when it comes to sitting on your ass on a cruise ship. This couple is really just getting absolutely plastered as a coping mechanism for whatever it is at home that is driving them out to sea 8 times a year. Stay away from them -- they seem enchanting at first, but soon you'll be overwhelmed with boozy hugs, uncomfortable nicknames, and a general feeling of regret.
3. The ports aren't exactly an authentic views of the countries you visit. You are hopefully well aware that mariachi bands won't wait to cheer you on whenever you walk into town, but your port cities will really try to convince you of this. It's like visiting the Disneyland version of a country, with all the uncomfortable racist feelings that go along. Are you helping the local economy and population, or are you perpetuating harmful colonial attitudes of the natives being "entertainment"? Tough questions - you'll probably need another margarita.
4. Speaking of drinks..booze is expensive. Really expensive. You are trapped on a boat with hundreds of people you don't know, where you entertainment for the night might be limited to dancing in the geriatric dance club. You'll need some strong drinks, and the cruise company knows it. So they'll make you sell your first born to pay for those margaritas. Luckily, cruises are quite good at hiding this from you, so you don't let it ruin your trip (they're very thoughtful). They'll let you just keep ringing up purchases on your cruise card, happily enjoying yourself while the money quickly drains from your account.