Dependency based boot sequencing is the default in Debian. The
insserv program is used by sysv-rc to order init.d scripts based on
dependencies declared in the LSB information within the scripts
themselves.

To monitor the boot sequence, the bootchart2 project is a good choice and
Debian packages are available.

Background info on alternative boot systems in Debian is available
from:
<URL:http://alioth.debian.org/docman/view.php/30730/38/debconf2-initscripts-bkg.pdf>
and more recently:
<URL:https://lists.debian.org/debian-devel/2012/03/msg00452.html>

Information on the LSB initscript comment convention is available from
<URL:http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/iniscrptact.html>,
<URL:http://wiki.debian.org/LSBInitScripts> and
<URL:http://wiki.debian.org/LSBInitScripts/DependencyBasedBoot>.

Information on an alternative approach to parallelism during boot is
available from
<URL:http://www-128.ibm.com/developerworks/linux/library/l-boot.html>.

A tool to detect cycles is available from Adam Jensen using the git
repository <URL:http://linuxguy.org/git/initcycle.git>.