There is a limit on the size of the root partition. Now, this makes sense in one way because it avoids the issue where Lilo can't read the kernel because it is beyond the area of the disk which is accessible by the BIOS. However, it is possible to create a big root partition but have a small partition mounted on /boot. The new installation system stops you doing this. I am aware that this is a somewhat debatable point, so I will raise it on the list as well. I for one, though, will find it very annoying if it gets into the final product. (If you do decide to put it in, it would be nice to have a way of overriding it, for example by stopping it applying to expert mode installations.)
Disk Druid merely throws out a warning. It allows you to do this, but you must later put in a smaller /boot partition. The documentation (and possibly the disk druid warning) could be a little better here though.