1. Nikon D80 costs more than Canon 400D. Nikon D80 costs about the same as Canon 30D (B&H: Nikon D80 $1000, Canon 30D $1050 after rebate). Therefore, when comparing functions/build quality, it is more appropriate to compare Nikon D80 with Canon 30D.
2. Now, D80 is an entry level DSLR, while Canon 30D (and its predecessor 20D) is used by many professionals (more wedding photographers use 20D/30D than 5D or 1D). Now, from commen sense, you should conclude that 30D should provide more features than 80D.
3. 1.6x (or 1.5x) crop cameras are used by a lot (probably more than 50% of those using Canon cameras) professionals. These cameras provide enough resolution (good enough for 30"x24" prints). Relatively few pros need weather sealing, or faster than 5 fps speed.
4. I also want to note that 8MP and 10MP does not make much difference for most people, because 8MP is about the maximum resolution of a Canon L type lens, and most people who buy the entry level DSLR (Nikon D80 or Canon 400D) will probably skip Canon's L lens. To get benefit of the 10MP resolution, you need to use a good prime lens at about f/8 (most likely in studio type setting), and print out the picture at VERY large size and look at it at close distance (which is unsually anyway). It will not make any difference if you only print out at 10"x16" or smaller.
5. Lens selection and full frame cameras. Canon carries a much larger selection of lenses, which gives you more room to move up in the future. Remember, 5-6 years ago, a professional DSLR has about 2 or 3 MP, and now, comsumer DSLRs are a lot better than the pro cameras just a few years back. So I believe in a few years, today's amature DSLR users will shoot full frame DSLRs instead. The expensive lenses you buy today (like 24-70mm f/2.8, 70-200mm f/2.8) will still function in the future.