Coffee Stain Studios leaves a nasty stain on their latest iteration of their TD/FPS series with Sanctum 2.
The tele-vator which allowed for quick and easy movement between key locations in each level has been removed and replaced with a new "feature" which requires you to run back all the way back to the end of your maze after every level in order to grab materials required to improve defenses at the front of your maze.
The side paths and alternate routes that made maze creating a fun strategic part of the original have all but been completely removed and replaced with singular uninteresting rectangular areas is almost all of the levels.
There is now a tower cap on each level which limits the number of towers that can be built. Towers are still limited by resources and upgrade costs, and there is no indication of a reason for this extra restriction.
Monsters can now attack the player and will frequently do so through walls, towers, and other objects that otherwise completely block player movement. Clever players may find ways to use this to their advantage as it is fairly easy to pull enemies away from their default pathing line.
Boss monsters now also have the ability to instantly destroy sections of your maze adding to the feel that this is much less a strategic tower defense game, and much more a first person shooter.
Players may now carry 2 types of guns (down from 3), none of which can be upgraded. This might actually be one of the few design changes that improved upon the original since new players will no longer enter multiplayer matches trying to be the guy that only upgrades his weapons without contributing to the teams overall defense with towers.
There have been some nice cosmetic changes, including the campaign mode comic style story panels and the introduction of new characters, but the game itself seems like a stripped down and restricted version of what it could have been.
TL;DR - If you haven't played the original, and are considering buying Sanctum 2, you should do a bit more research and consider buying the original Sanctum instead. If you enjoyed the original, stay as far away from the sequel as you can.