There are things I hate about the first day of class. I hate walking into a room full of strangers; I find students in the anonymous collective kind of intimidating. Once I've taught them a few times and start to be able to sort them out into individual personalities, that goes away (usually; there is occasionally the odd student whom I find intimidating, personally, but that's rare). But the first day they're all staring at you and I'm always convinced they're thinking, "Oh God, who is THIS dork?"
I also hate the artificiality of trying to get them talking and engaged when we haven't covered any material yet. The standard advice is that you need to set the tone for the term from the first day - if you want students to talk and share ideas, then you need them to do that the first day (none of this "let's read through the syllabus and go home"!). And I get that, and I kind of agree with it, but I teach something incredibly content-based, with content that very few students know much about before walking into my classroom. I usually do a "What do you think of when you think of the Middle Ages?" discussion on the first day, but really, that only goes so far (it segues into a little random lecturette on the humanist invention of the Middle Ages and the Pre-Raphaelite/Arts & Crafts romanticization of the pre-industrial, but if it turns into me lecturing that defeats the purpose of getting students to talk). I'm fine with getting students to talk about something we've already read, but that first day is kind of a drag.
But you know, I do like the shiny newness of the first day sometimes. You get students who are excited about the subject and haven't been dragged down by the reality of actual assignments yet. They smile and nod and even kind of laugh at your lame jokes, because there's that air of hope, of potential, that this - this! - will be the cool class that college is supposed to be filled with, the class in which you learn a ton but it's so fascinating and fun that you don't even realize you're learning, the class in which the papers practically write themselves because the reading for them is like play not work, the class in which you feel at once incredibly smart and yet also like you're being challenged and learning and growing. I don't think I've ever taught that class, but on the first day, we can all hope that this - this will be it.