Trust me, you'll love the bike. I came off an XR400 and riding the '04 450 EXC is liking riding in a different universe. The suspension is amazing stock and the power is great. Don't worry about the power. There's plenty there if you need it, but it doesn't really try to get away from you much.
I thought the seat was terrible on my first ride (just like most people say) but I've honestly gotten used to it. Don't think I'll change it.
I put a Flatlands skidplate on it, clutch slave cylinder guard, Cycra Pro-Bend hangguards, a GPR 2.0 under-the-bars stabilizer and that's pretty much it. Ride it a few times and then realize that when you put a GPR stabilizer on it, it'll be even MORE fun to ride!
As far as break in procedures, I got lots of advice when I asked the same question about my bike (I got it on Oct 3). I heard everything from baby it a lot to ride it wide open on the first ride. I just couldn't bring myself to running it very hard on the first ride. Before I ever rode it, I ran through a couple of heat cycles in the garage. Just start it up, get the temp up a little bit and shut it off. As a rule of thumb, the bike is pretty near operating temperature when the radiator gets warm. Once it's warm, shut it off and let it sit and completely cool down. Once it's cool, start it back up and warm it up again. I did this 3 times before I ever rode the bike. Just be careful because it will warm up again quickly. If you don't keep an eye on it, it will overheat and you'll lose some radiator fluid.
The following day I rode it for the first time. Just take it easy and use maybe 50%-60% of the throttle. I rode for about 15 minutes and then parked it to let it cool off a little bit. I think I did that 3 or 4 times the first day. On the next ride, I was probably at about 75% of throttle. I put about 2 hours on it that day and then changed the oil when I was done. The next ride I opened it up and discovered how great the bike really is.
I'm sure I was waaaay too picky on my break-in, but I wanted to make sure I was gentle with my new baby. Hope that helps.
Oh yeah, not sure if you're in California, but the bikes come really lean from the factory. My first step was dropping the clip two positions on the needle. If your headpipe starts turning blue on your first few rides you'll know it's running a little lean. You can also look into the JD Jetting Kit. I don't have it yet, but a lot of guys are recommending it in CA.
Have fun with it!