I had just finished the fifth season of this show when for some reason I decided to jump back and watch the pilot episode. Made 2 or 3 years before the show returned with its first season of episodes, the pilot is very rough around the edges but is interesting to see for the roots of the design and fundamental ideas that would later become the show. It is very difficult for me to watch it in hindsight, or rather difficult to draw a conclusion based solely on the pilot due to how much of the later show I have watched. I am quite jealous of those here who wrote IMDb comments in 2007/08 about how great the pilot was, considering that time (and studios and audiences) haven proved them to have called a good one from this series of random cartoons.
To me it is overly rough, the animation is very basic compared to the very polished show, and indeed the tone is probably too much "yeh! adventures!" but it works and there are nice asides and moments of slight randomness where you can see the tone of the humor. Likewise some of the delivery has a nice down-to-earth feel to it, which sits nicely with the other material pushing the show "up" as it were – specifically I liked the "normal guy" personae of a large elemental fireball asking for directions. The character creation also indicates a lot of good ideas; Ice King has the comic trimmings of his drum kit, while the rainicorn shows color and invention.
So to watch this some 7 years after it was made, and in the shadow of a show that is on its 6th season and is of a consistently high standard, it is hard not to see it as a curio of a sort, rather than a part of the show itself. As such I was glad I watched it, but after a fifth season full of invention, wit, fun, horror, character development, and fantastical moments, it is a big ask of the pilot to expect it to be more than this; although as such it is well worth watching.