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Features
The ideas are borrowed from other packages, and some of them are re-implemented in a different way (like cache). A selected list of features include:
faithful output: using evaluate as the backend to evaluate R code, knitr writes everything that you see in an R terminal into the output by default, including printed results, plots and even warnings, messages as well as errors (they should not be ignored in serious computations, especially warnings)
a minor issue is that for grid-based graphics packages like ggplot2 or lattice, users often forget to print() the plot objects, because they can get the output in an R terminal without really print()ing; in knitr, what you get is what you expected
built-in cache: ideas like cacheSweave but knitr directly uses base R functions to fulfill cache and lazy loading, and another significant difference is that a cached chunk can still have output (in cacheSweave, cached chunks no longer have any output, even you explicitly print() an object; knitr actually caches the chunk output as well)
formatting R code: the formatR package is used to reformat R code automatically (wrap long lines, add spaces and indent, etc), without sacrificing comments as keep.source=FALSE does
more than 20 graphics devices are directly supported: with dev='CairoPNG' in the chunk options, you can switch to the CairoPNG() device in Cairo in a second; with dev='tikz', the tikz() device in tikzDevice is used; Could anything be easier than that? These built-in devices (strictly speaking, wrappers) use inches as units, even for bitmap devices (pixels are converted to inches by the option dpi, which defaults to 72)
even more flexibility on graphics:
width and height in the output document of plots can be additionally specified (the fig.width option is for the graphics device, and out.width is for the output document; think out.width='.8\\textwidth')
locations of plots can be rearranged: they can either appear exactly in the place where they are created, or go to the end of a chunk together (option fig.show='hold')
multiple plots per code chunk are recorded, unless you really want to keep the last plot only (option fig.keep='last')
R code not only can come from code chunks in the input document, but also may be from an external R script, which makes it easier to run the code as you write the document (this will especially benefit LyX)
for power users, further customization is still possible:
the regular expressions to parse R code can be defined, i.e., you do not have to use <<>>= and @ or \Sexpr{}; if you like, you can use any patterns, e.g., %% begin.rcode and %% end.rcode
hooks can be defined to control the output; e.g. you may want to put errors in red bold texts, or you want the source code to be italic, etc; hooks can also be defined to be executed before or after a code chunk, and there are infinite possibilities to extend the power of this package by hooks (e.g. animations, rgl 3D plots, …)
Lots of efforts have been made to producing beautiful output and enhancing readability by default. For example, code chunks are highlighted and put in a shaded environment in LaTeX with a very light gray background (the framed package), so they can stand out a little bit from other texts. The reading experience is hopefully better than the verbatim or Verbatim environments. The leading characters > and + (called prompts) in the output are not added by default (you can bring them back by prompt=TRUE, though). I find them really annoying in the output when I read the output document, because it is so very inconvenient to copy and run the code which is messed up by these characters.
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