You are right that if Java's byte code specification is there, we can
develop a compiler for some other language over Java platform also. But Java's
bytecode is just suitable for Java language. For example, Microsoft .NET's IL is
a separate language, you can even program using that, and that language has got
many more features than you might have expected, it supports a lot, but any .NET
compiler only uses a subset of those features, for example, MS.NET IL is capable
of providing operator overloading, this is used by C# but J# does n't use that,
so it is fair to call ..NET is Language independent.
Yes languages needed to be modified, for them to be compatible with .NET
platform, but take the case of some language like Perl, it is a good programming
language but it has no available libraries, at least not as many as C or C++
has, if the developers of Perl made small changes such that it is compatible
with .NET platform, then you see whole .NET library can be used by Perl
developers, what else they want. It is a common problem for man, to lose
something to gain something, but the thing gained in this case is far more
valuer than the thing lost.
And regarding your view about ..NET's JIT compiler. yes there are many free
JIT compilers for Java also, even the JRE has that facility, but the facilities
offered by .NET's JIT are not offered by any of the Java JIT compilers. .NET's
JIT can save the code once compiled, in memory and then use the same compiled
code, when the same sort of code is found. And also, .NET's has a facility to
save the Native compiled code in a file so that when the program is running,
that compiled code is used instead of compiling again, making the program run at
full speed.

.NET is a good platform, it has got everything Java has and a lot more. And
for everybody in this group who are expecting me to be somewhere related to
Microsoft, Iam a pure Java developer, and has n't even developed a single
program on .NET platform.