Always raise plus sign in text

You could make + active and raise itself in text mode and not in math, but something would break, it is quite hard to catch all cases of \dimexpr \parindent + 5pt\relax and ensure you don't add a \raisebox mid-expression.

I would use a new command for it, \+ isn't defined by default so:

\documentclass{article}

\newcommand\+{\raisebox{0.25ex}{+}}
\begin{document}

NNLL+NNLO


NNLL\+NNLO

\end{document}

I'm not sure that 0.25ex is the right choice: it actually makes the + sign to be slightly higher than a capital letter.

Using different fonts might also make the situation even worse. For instance, with Times you'd get

because here the + sign sits on the baseline. Can we make the raising independent of the font? Yes: a bit of algebra shows that we need to raise the symbol by half the sum of a capital letter, minus the height of + plus the height of +.

Using David's idea:

\newcommand{\+}{%
  \raisebox{\dimexpr(\fontcharht\font`X-\height+\depth)/2\relax}{+}%
}

Here's the output with Times

and with Computer Modern

Here's a visual proof of the statement about the height. The first + is with my definition, the second is raised 0.25ex. Just look at the top, because at the bottom TeX always uses the baseline.