From VarLogDistUpgradeMain.log we can see the following decisions being made:

2015-10-20 12:03:36,420 DEBUG Foreign: 
2015-10-20 12:03:36,420 DEBUG identifyObsoleteKernels()
2015-10-20 12:03:44,628 DEBUG identifyObsoleteKernels found 'set()'
2015-10-20 12:03:44,628 DEBUG forced_obsoletes: ['desktop-effects', 'gnome-app-install', 'policykit-gnome', 'gnome-mount']
2015-10-20 12:03:45,909 DEBUG Unused dependencies: linux-image-generic libavutil54 libavcodec56 libx264-142 linux-headers-4.2.0-16 libilmbase6 linux-headers-generic libswscale3 libavformat56 libavresample2 libdirac-encoder0 libopenexr6 linux-image-4.2.0-16-generic linux-headers-4.2.0-16-generic linux-image-extra-4.2.0-16-generic thermald
2015-10-20 12:03:45,909 DEBUG remove_candidates: '{'libapt-pkg4.12', 'policykit-gnome', 'libedata-book-1.2-20', 'libswscale3', 'libufe-xidgetter0', 'libgdata19', 'libecal-1.2-16', 'xfonts-mathml', 'linux-image-extra-4.2.0-16-generic', 'thermald', 'libgnome-bluetooth11', 'libboost-iostreams1.55.0', 'libplist2', 'libavutil54', 'linux-headers-3.19.0-30', 'libavcodec56', 'libpoppler49', 'libwps-0.3-3', 'linux-headers-3.19.0-30-generic', 'qtdeclarative5-ubuntu-web-plugin-assets', 'linux-headers-generic', 'gnome-app-install', 'libboost-system1.55.0', 'libxp6', 'linux-headers-4.2.0-16-generic', 'libmircommon3', 'linux-image-3.19.0-30-generic', 'libebook-1.2-14', 'libapt-inst1.5', 'libvpx1', 'linux-headers-4.2.0-16', 'libnettle4', 'libcamel-1.2-49', 'gnome-mount', 'libboost-filesystem1.55.0', 'libopenexr6', 'libinput7', 'libgnome-control-center1', 'libebook-contacts-1.2-0', 'librhythmbox-core8', 'libvncclient0', 'libexiv2-13', 'libicu52', 'libhogweed2', 'webaccounts-extension-common', 'libx264-142', 'xul-ext-webaccounts', 'libmediaart-1.0-0', 'linux-image-generic', 'libedata-cal-1.2-23', 'libilmbase6', 'libavformat56', 'libavresample2', 'libdirac-encoder0', 'libgphoto2-port10', 'desktop-effects', 'linux-image-4.2.0-16-generic', 'libedataserver-1.2-18', 'libebackend-1.2-7', 'linux-image-extra-3.19.0-30-generic', 'libxcb-util0'}'
2015-10-20 12:03:45,910 DEBUG Start checking for obsolete pkgs
2015-10-20 12:03:52,626 DEBUG skipping running kernel pkg 'linux-headers-3.19.0-30-generic'
2015-10-20 12:03:52,626 DEBUG 'linux-headers-3.19.0-30-generic' scheduled for remove but not safe to remove, skipping
2015-10-20 12:03:54,526 DEBUG skipping running kernel pkg 'linux-image-3.19.0-30-generic'
2015-10-20 12:03:54,526 DEBUG 'linux-image-3.19.0-30-generic' scheduled for remove but not safe to remove, skipping
2015-10-20 12:04:03,470 DEBUG skipping running kernel pkg 'linux-image-extra-3.19.0-30-generic'
2015-10-20 12:04:03,470 DEBUG 'linux-image-extra-3.19.0-30-generic' scheduled for remove but not safe to remove, skipping
2015-10-20 12:04:03,745 DEBUG Finish checking for obsolete pkgs
2015-10-20 12:04:03,821 DEBUG The following packages are marked for removal: xfonts-mathml libboost-system1.55.0 libebook-contacts-1.2-0 libavformat56 libavresample2 libplist2 linux-headers-generic libmediaart-1.0-0 libgdata19 xul-ext-webaccounts libexiv2-13 libebackend-1.2-7 libicu52 linux-headers-4.2.0-16-generic libilmbase6 libboost-filesystem1.55.0 libxp6 libufe-xidgetter0 libwps-0.3-3 libboost-iostreams1.55.0 linux-image-extra-4.2.0-16-generic libapt-inst1.5 libvpx1 libpoppler49 linux-image-4.2.0-16-generic thermald libgnome-bluetooth11 libgnome-control-center1 libedata-book-1.2-20 libmircommon3 libgphoto2-port10 libswscale3 qtdeclarative5-ubuntu-web-plugin-assets linux-headers-4.2.0-16 libcamel-1.2-49 libapt-pkg4.12 libhogweed2 libdirac-encoder0 libxcb-util0 libopenexr6 libecal-1.2-16 libebook-1.2-14 libnettle4 webaccounts-extension-common libedataserver-1.2-18 linux-headers-3.19.0-30-generic libavcodec56 libx264-142 libinput7 librhythmbox-core8 libedata-cal-1.2-23 linux-image-generic libvncclient0 linux-headers-3.19.0-30 libavutil54