Problem with \leq displaying as \leqslant

In this regard mathabx is not “harmless” as its purpose is to provide and changes lots of symbols. In particular, it changes the \le symbol (see page 6 of the manual). It also provides \varleq, which looks closer to the Computer Modern \le.

Caramdir provided a very good TeXnical answer; let me give a short aesthetical one: I'd just be happy and stick with the \leqslant version since it looks much better, in my opinion. This is of course personal taste, but I like that it's closer to what I do in handwriting.

As a note to whoever might find it useful, the Euler math (package euler or eulervm) also slants the less-than-equal or greater-than-equal symbols. This is used e.g. in classicthesis which I am using. I just spent a long time trying to figure it out as I haven't included mathabx in my code. Duh ...

If you still like to use your font (e.g. the above-mentioned euler) but want the classic less-equal symbol, you can pull it out, by putting this in your preamble:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% taken from mathabx.sty. Version: May 18, 2005.
% Author: Anthony PHAN.
% matches the ``mathabx'' family.
% LaTeX input/style file
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\DeclareFontFamily{U}{matha}{\hyphenchar\font45}
\DeclareFontShape{U}{matha}{m}{n}{
      <5> <6> <7> <8> <9> <10> gen * matha
      <10.95> matha10 <12> <14.4> <17.28> <20.74> <24.88> matha12
      }{}
\DeclareSymbolFont{matha}{U}{matha}{m}{n}
\DeclareFontSubstitution{U}{matha}{m}{n}
\DeclareMathSymbol{\varleq}{3}{matha}{"A8}
\DeclareMathSymbol{\vargeq}{3}{matha}{"A9}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

And maybe also make it used:

\renewcommand{\leq}{\varleq}
\renewcommand{\geq}{\vargeq}

Likely, there are easier ways. Maybe it would be a better idea to get it out of the default font.