Shooter multiplayer modes live or die by two things: partners and options. The importance of having partners varies from game to game. Someone who's really skilled at Call of Duty or Halo can run solo and still top the leaderboards but for co-op or objective modes having a team is pivotal. Options are more important across the board because they keep multiplayer fresh. Maps get stale. Game modes get mastered. Mixing different toolsets like classes, equipment and weapons gives players a way to challenge themselves;the successful way to play as an Assault class character is not how you play as a Sniper class character.
Unlocking options is usually tied to progression measured by experience points. If you get X number of experience points you unlock the next level, and with that level comes the ability to use some new weapons, or equip some new gear, or take advantage of new abilities. If a player sticks with the game they will eventually unlock everything, and achieving those goals provides another layer of challenge.
Mass Effect 3's multiplayer has stuck both the accrual of options and the challenge of using them behind a randomization system and an in-game purchase monetization model the likes of which is normally seen only in free-to-play games. And the more content BioWare and Electronic Arts release for ME3 multiplayer, the more these choices feel not only like a disincentive to bother with multiplayer anymore, but also like a petty method to try and make a few extra bucks that is getting in the way of an otherwise brilliant multiplayer mode.
It is factually correct to say the Rebellion Pack DLC introduced six new character classes, two new maps, three new weapons and a new game type for Mass Effect 3's multiplayer mode. It might be more accurate to say the Rebellion Pack adds two new maps and a new game type for sure, and might add the rest of the new content if you get lucky rolls of the dice.
Mass Effect 3 multiplayer is basically Horde mode from Gears of War. In the default, baseline game mode up to four players have to survive ten waves of enemies and then hold an extraction point for two minutes until a shuttle arrives to pick them up. There are three levels of difficulty: Bronze, Silver and Gold. Gold matches are absolutely brutal.
All six characters classes from the campaign are available in the multiplayer, with human characters as the default choices. There are also alien variants with unique abilities to mix things up. Where the human character classes are available from the very beginning, the alien types are unlocked randomly through Reinforcement Packs.
Reinforcement Packs are akin to packs of Magic: The Gathering cards. You pay X number of credits for a Pack and get five random items. They can be single-use pieces of equipment, new weapon unlocks or weapon upgrades, experience points and character customization options for character classes, or unlocks for new character classes like the alien variants. The more expensive the variant of Reinforcement Pack, the greater your chances of getting Rare items. A Reinforcement Pack that costs 99,000 credits usually comes with two guaranteed Rare items, for example.
Credits are earned by completing objective-based waves during matches, and the higher the difficulty and the faster you achieve the objectives, the more money you earn. A Gold-level match usually takes about 20 minutes to complete and can net a player around 70,000 credits, so for every 40 minutes or so of multiplayer at its most punishing, difficult setting a player can afford a chance at getting two Rare items they're looking for.
I cannot tell you how many times I've spent that 99,000 credits and received absolute garbage from the Pack I bought. Weapon powerups for weapon classes I don't use. Equipment I really don't need. Experience point bundles for character classes I've long since maxxed out at Level 20, which means those items literally go to waste. And once you get an item from a Reinforcement Pack you're stuck with it. You can't sell items back for credits and you can't trade them with other players.