Adept- Pure biotic. Like the Engineer and Sentinel, the Adept stands to benefit dramatically from ME3's cooldowns vs weight system. The Adept's game plan in ME3 seems tailor made to exploit warp detonations. With Singularity, Warp, Pull, Throw, and Shockwave, the Adept has endless ways to create detonations. The Adept's drawback is a poor weight capacity, so one-weapon caster-heavy builds are likely to be the most useful.
Engineer- Pure tech. Engineers enjoy a massive buff from ME2. With Sentry Turret and Combat Drone an Engineer has access to two powerful, fast-spawning mobile weapon platforms that they can create almost anywhere in the area of battle, turning essentially any battle into an unfair fight in favor of Shepard. If that wasn't enough, the Engineer can also tear through any enemy protection with Incinerate and Overload along with hacking robots through Sabotage.
Infiltrator- Combat + tech hybrid. The Infiltrator can capably punch through all enemy protections with their ammo types and Incinerate, and with cloak becomes a lethal combatant at both close quarters and long range. The Infiltrator's long-range abilities, in particular, have been emphasized in ME3, with the final rank of Tactical Cloak offering a ridiculous 40% bonus to sniper rifle damage when firing from cloak. Shepard's newfound mobility and unlimited sprint also greatly help Infiltrators in one of their main problems from ME2: displacing from cover to cover in time while cloaked. A maxed Cloak also lasts significantly longer than ME2's, allowing the class to be incredibly effective at lining up lethal cloaked sniper headshots.
Sentinel- Biotic + tech hybrid. Pure Sentinel players can now fully streamline their build along the two major Sentinel types: the "artillery" caster type, and the "tank" assault type. Caster Sentinels can obliterate all enemy protection with Warp and Overload, while maximizing their power cooldowns; Assault Sentinels can monstrously buff their tech armor to allow for prolonged close combat. Like their ME2 version, Sentinels remain the jack of all trades with a power for any situation. However, their main constraint (limited weapon selection) has now been removed. This makes Sentinels the most purely customizable class in the game.
Soldier- Pure combat. Soldiers now have more versatile combat options with the expanded abilities of Concussive Shot, and the ability to rip all forms of enemy protection with their varied ammo types. With the new power cooldown and weight capacity system, assault-rifle-only Soldier builds with drastically reduced Adrenaline Rush cooldowns are a tantalizing (if frighteningly overpowered) combination. However, the Soldier's bread and butter remains running to the sound of the guns and blasting through the opposition with a nonstop barrage of lead. Shredder spec for Concussive Shot adds yet another massive damage buff to the Soldier's damage output against organic enemies, which make up the majority of opposition to begin with.
Vanguard- Combat + biotic hybrid. Excels at vicious close-quarters battle. A properly played Vanguard is nothing less than a pure killing machine. Two of the most pressing problems for ME2 Vanguards (mid-range offense, and staying alive during Charge cooldowns) have been solved right off the bat with ME3's free weapon selection and increased character mobility. Charge itself has been substantially buffed, now hitting for heavy damage on impact, whereas in ME2 it only caused a negligible amount of physics-based damage. Like the Infiltrator, the Vanguard's entire game revolves around its class power, just with a polar opposite combat paradigm. While Infiltrators rely on long-range headshots, the Vanguard lives to charge into the enemy's face and rearrange it with the biggest shotgun available. Pull and Nova give the Vanguard additional options for ranged and close-quarters engagements.