One of the first issues that a Linux user will run into when installing OpenSolaris is the fact that it does not support partitions inside an extended partition; if you install OpenSolaris on an extended partition, it will erase all the partitions inside it. This is important to mention, as many Linux distributions install inside extended partitions by default.
Related to this is OpenSolaris' file systen, ZFS. The Linux kernel does not have support for ZFS, and as such, if you want to exchange data between the two on the same machinem you'll have to use FUSE. The article erroneously states that this is because the FSF does not consider the CDDL "free enough", but the actual reason is that the CDDL is simply not compatible with the GPL. If the CDDL is not free enough, then neither is the GPL, as you could argue that the CDDL is less restrictive than the GPL.
The version of GRUB shipped with OpenSolaris has been modified to support booting ZFS partitions (obviously), but this support is not present in the GRUB versions shipped with Linux distributions. As such, it is important to note that if you install Linux after installing OpenSolaris, you need to make sure not to erase OpenSolaris' GRUB.
While Linux' hardware support is wider than that of OpenSolaris, the latter does benefit from having a static driver interface. Where in Linux hardware support might actually break as time goes by, 10 year old Solaris drivers will still work today. There's also a Device Detection Tool which will tell you if your hardware is compatible with OpenSolaris.
The article also details that OpenSolaris will run slower than comaprable Linux distributions, and that the number of applications to choose form is quite limited compared to what Linux distributions generally have to offer.
The article is an interesting read, and further details a number of OpenSolaris-specific features, such as ZFS and Zones.