important parts of the api that are missing
in no particular order
------------------------------------------

missing apr_buckets api:
destroy, delete

tied filehandle interface:
 -EOF, TELL, SEEK
 -READLINE - proper implementation (see comment in Apache::compat)
 need to attempt to fix that interface so it'll work with IPC::Open* family  

$r->finfo:
need apr_finfo_t <-> struct stat conversion (might already be there,
haven't looked close enough yet)

$r->chdir_file:
not safe for threaded environments.  should at least unshift @INC with
dirname $r->filename.  consider overriding open() to resolve relative
filenames.

$r->child_terminate:
haven't looked into this, but clearly not an option with threaded
mpms.  might consider knocking off the current PerlInterpreter
instead.

$r->log_reason:
should be simple, see modperl_log.h

Apache->server_root_relative:
needs to default to current pool (pconf at startup, r->pool at request
time)

APR::URI
missing $uri->rpath

Apache::Util::*
escape_path() - (was escape_uri) can be changed in apache not to require a
pool

size_string() - can we use apr_strfsize ?

escape_html() -  consider jeff baker's more robust implementation of
my_escape_html(), which should probably be made in apache-2.0 itself

ht_time() - now requires a pool, temporarily renamed format_time for
1.x ht_time compat.  should we just leave as is or see if ht_time can
be changed to not require a pool?

$r->document_root:
cannot currently be modified.  requires locking since it is part of
the per-server config structure which is shared between threads

$r->send_fd:
need to figure out howto map PerlIO <-> apr_file_t
at the moment $r->send_fd is implement in Apache::compat, functions,
but does not have the performance benefits of ap_send_fd()
however, $r->sendfile is a new function that opens the file for you
and calls ap_send_fd() underneath. ap_send_fd() in APR doesn't work 
with fd's which aren't files and of unknown length, therefore it cannot 
be used for implementing 1.x compatible send_fd.

Apache::{Server,Process} classes:
require mutex lock for writing

Apache->import:
not yet implemented, required for exit/warn overridding

$r->{get,set,push}_handlers:
need to deal properly with modification of the current handler phase
we're running.

Apache->request:
need to deal with subclass objects which are not a request_rec
(e.g. HASH ref)

$r->cgi_header_out:
anything in 1.x land actually using it?

$r->slurp_filename:
optimized version not yet implemented (compat version exists in
Apache::compat)

Apache->unescape_url{_info}:
not yet implemented.  should be moved to Apache::Util

mod_perl::import():
not yet implemented

Apache::FakeRequest:
since APR can be used outside of httpd, and we can alloc request_rec
and similar structures, it should be possible to bootstrap an
inside-httpd interface and outside-httpd interface.  its not really
worthwhile looking at until APR actually installs its *.so or *.a
libraries somewhere apxs can find them.  and, there's a bunch of util
functions (e.g. URI stuff) that is supposed to move from httpd into
apr-util.