Why is BASIC bad?
(1) It encourages poor programming practices by
a) not including good code block semantics.
b) not supporting classes
c) not separating library (or OS) calls from language primitives
d) having no proper concept of scope
e) not having a standard
(2) Being a suboptimal interpreted language - I remember that CLS was around 100 times slower than a Z80 routine.
Of course, you may be thinking of modern BASIC implementations - well, that's a different thing altogether.
The BASICs (eg on the Spectrum, C64, etc) I knew were just rubbish.
I guess you could argue that it's an implementation thing - but actually, it's hard to go wrong with something like Java, or C. I remember some LISP interpreters were pretty slow also..