Most of the style hints in the Lisp FAQ apply.

When porting the system, I would really prefer code which factors
dependencies into a set of interface functions and constants and
includes implementations of the interface for the different systems.
Patches which require conditional compilation (like all the old
#T+HPUX or #T-X86 tests in the sources inherited from CMUCL) might be
accepted if they're simple, in hopes of factoring out the differences
more cleanly later, but even if accepted, such code may not be
maintained for very long.

grammatical fussiness:
  Phrases are not capitalized.
  Sentences are capitalized.
  Periods terminate sentences.
  Periods separate phrases from succeeding sentences, e.g.
    ;;; the maximum number of transformations we'll make before
    ;;; concluding we're in an infinite loop and bailing. This can
    ;;; be changed, but it is an error to change it while we're
    ;;; solving a system.
    (defvar *max-n-transformations* 10)
  Lisp in comments is capitalized.

usage fussiness:
  Function documentation can be a description of what the function
    does, e.g.
	;;; Parse the arguments for a BDEFSTRUCT call, and return
	;;;   (VALUES NAME DEFSTRUCT-ARGS MAKE-LOAD-FORM-FUN BDEFSTRUCT-STYPE),
	;;; where NAME is the name of the new type, DEFSTRUCT-ARGS is the
	;;; munged result suitable for passing on to DEFSTRUCT,
	;;; MAKE-LOAD-FORM-FUN is the make load form function, or NIL if
	;;; there's none, and BDEFSTRUCT-SUPERTYPE is the direct supertype
	;;; of the type if it is another BDEFSTRUCT-defined type, or NIL
	;;; otherwise.
	(defun parse-bdefstruct-args (nameoid &rest rest)
	  ..)
    or a remark about the function, e.g.
	;;; a helper function for BDEFSTRUCT in the #+XC-HOST case
	(defun uncross-defstruct-args (defstruct-args)
	  ..)
    If you're talking about what the function does, ordinarily you
    should just say what the function does, e.g.
	;;; Return the first prime number greater than or equal to X.
	(defun primify (x) ..)
    instead of telling the reader that you're going to tell him what
    the function does, e.g.
	;;; PRIMIFY returns the first prime number greater than or 
	;;; equal to X.
	(defun primify (x) ..)
    or 
	;;; When you call this function on X, you get back the first
	;;; prime number greater than or equal to X.
	(defun primify (x) ..)

In general, if you can express it in the code instead of the comments,
do so. E.g. the old CMUCL code has many comments above functions foo
that say things like
	;;; FOO -- interface
If we were going to do something like that, we would prefer to do it by
writing
	(EXPORT 'FOO)
(Instead, for various other reasons, we centralize all the exports
in package declarations.) The old "FOO -- interface" comments are bad
style because they duplicate information (and they illustrate one
of the evils of duplicating information by the way that they have
drifted out of sync with the code).

There are a number of style practices on display in the code
which are not good examples to follow:
  * using conditional compilation to support different architectures,
    instead of factoring the dependencies into interfaces and providing
    implementations of the interface for different architectures;
  * in conditional compilation, using a common subexpression over and
    over again, e.g. #+(OR GENGC GENCGC), when the important thing is
    that GENGC and GENCGC are (currently) the GCs which support scavenger
    hooks. If you have to do that, define a SCAVHOOK feature,
    write #+SCAVHOOK in many places, and arrange for the SCAVHOOK feature
    to be set once and only once in terms of GENGC and GENCGC. (That way
    future maintainers won't curse you.)
  * putting the defined symbol, and information about whether it's 
    exported or not, into the comments around the definition of the symbol;
  * naming anything DO-FOO if it isn't an iteration macro
  * exposing a lot of high-level functionality not in the ANSI standard
    to the user (as discussed above)
  * not using a consistent abbreviation style in global names (e.g. 
    naming some things DEFINE-FOO and other things DEF-BAR, with 
    no rule to determine whether the abbreviation is used)
  * using lots of single-colon package prefixes (distracting and hard
    to read, and obstacles to reaching package nirvana where 
    package dependencies are a directed acyclic graph) or even
    double-colon package prefixes (hard to understand and hard
    to maintain). (One exception: I've sometimes been tempted to
    add a CL: prefix to the definition of every CL symbol (e.g.
    (DEFUN CL:CADDDR (..) ..) as reminders that they're required by
    ANSI and can't be deleted no matter how obscure and useless some
    of them might look.:-)
Most of these are common in the code inherited from CMUCL. I've
eliminated them in some places, but there's a *lot* of code inherited
from CMUCL..