The game is slightly biased towards the Isles (and Norway!) simply based on the start date.
Lots of interesting things going on all over the map, but I like the Isles for three reasons: 1) I live in the US, so I know more about English medieval history than anywhere else in Europe (you wanna talk Renaissance, its a different story, but then that's also a different game). 2) There is a lot of scope for expansion without a lot of interference from too powerful neighbors. No matter where you start, if you start small and independent like I do, you are going to have to expand in the opposite direction from the HRE and the ERE. Not an issue in the Isles. 3) There are a lot of independent rulers to choose from there, and I already have a good understanding of the political situation. That being said, once I have formed Britannia (from Brittany of course!) in my current game, I think I will move on to Central Europe. It is time I stopped hiding from the Kaiser!
BTW, my first game was based on the Dublin walkthrough and my second was Barcelona (I wanted to get my reconquista on!) and though Dublin taught me the basics, Barcelona is where I really learned to play! Keep coming back to the Isles though.