There is a number of reasons why people dislike Solaris.
1) Change. In general people dislike change. Change requires re-learning and retraining. Old system administration practices no longer work. Commands have been replaced by other commands, some commands behave differently. And they ask why the change was necessary. SunOS 4.x worked for them.
2) Lack of migration support. Sun did not provide a lot of tools to ease migration. Many applications wouldn't run in the binary compatibility mode. The source compatibility mode was probably compatible with some OS, but it certainly wasn't SunOS. Lots of public domain and third party stuff that was needed wasn't immediately available for Solaris. NIS+, buggy, resource hungry and unstable replaced NIS in incompatible ways.
3) Missing functionality. When people migrate, at first they don't tend to notice new functionality. Instead, they stumble upon missing functionality such as screenblank, clear_colormap and the like (but see 3.23). And perhaps worst of all, no C compiler, not even a crippled one.
4) Slow and buggy. The initial Solaris releases didn't perform at all well and were extremely unstable. This has improved drastically, with Solaris 2.5 being stable and quick, even without many patches.