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 Commands have been replaced by other commands,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 3.26). 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.