Shin Megami Tensei: Devil Survivor gets even deeper when you factor in the customization elements. There are a dozen playable characters and 100+ demons to combine into teams. Additionally, each can carry up to seven skills, seting up a massive number of party configurations.
Skills are earned through Skill Crack, a system that lets you steal skills from enemies during battle. Before battle, you assign the skills you want characters to steal and if they can defeat the skill-bearer, they learn the skill. Once the battle is over, you can equip the learned skill. Skill Cracking removes a lot of tedious grinding, though it also places another layer of strategy to combat. Learned skills are unique to a character, so you have to keep player growth/ progression in mind at all times.
Depending on how well you perform in a fight, you'll earn Macca (demon "money"). You'll always earn a base amount of Macca, though pulling off combinations and other performance goals (teamwork, damage taken/ done...) will earn bonuses. Macca is used to power up your team and serves as currency when "buying" new demons via an in-game auction system. I can see where it would throw off the game's balance, but a real online auction house would have been a cool addition.
Demons level up alongside their masters, though you'll have to use the Fusion system to unlock their full potential or to create some of the game's better demons. One welcome addition to the SMT series is a searchable fusion database. It takes all the guesswork out of creating fusions. Several recognizable demons are present along with a few new ones.
Certain games deserve big sales numbers and Shin Megami Tensei: Devil Survivor is one of them. The difficulty will likely scare away a few interested players, but it shouldn't be a determining factor. Though difficult, the game is fair. You may not blast through the game, but the amount of content and branching paths will pull you back for more. If you're a fan of monster-collecting games or strategy games, Devil Survivor is a must-own.