I first got into Dream Theater around the time A Change of Seasons came out. For a few weeks, I didn't even realize that the keyboard player wasn't the same as on Images and Words and Awake, the other two albums I had. I'd sat down with the (as far as I knew) first two albums and read all the credits and gotten into them, but when I got ACOS, I just put it on and dug into it. One day I finally got around to reading the credits, and just said "Oh, they have a new keyboard player. Huh."
I was still adjusting my ears and brain to the whole Prog Metal concept, which was still very new at the time. With Falling Into Infinity, there were a lot of parts that I thought were guitar, and didn't realize were keyboards until I watched Five Years in a LIVEtime. Derek's lead sounds then (and now to an extent) do seem to mimic a guitar in some ways. There's no question that his approach to soloing and other lead lines is similar to how a guitarist might play the same parts. Being a keyboard player myself, I've always had mixed feelings about that. In principle, I feel like keyboards should be distinct, just as the bass and guitar are distinct, even if they're doubling parts or whatever. I mean, why have a keyboard player if he's just gonna play "guitar-like" stuff? But Metal has always been about guitars, so if someone figured out a way to integrate keyboards into Metal, then I was glad to see it happen.
The basic formula to me, at the time, was Metal + keyboards = Prog Metal. Obviously that's an oversimplification, but DT was breaking new ground, pretty much writing the rules as they went along. Kevin's approach was more "traditional" in terms of keys, and was great, but I really liked what Derek brought to the band. I thought it was a step forward in the evolution of the band.