So I've recently downloaded and played the Mass Effect 3 Demo. First off, I have to say I'm quite impressed. Mass Effect 3 has improved upon the formula of Mass Effect 2. I can't really speak for the RPG elements, since the Demo is quite short and focuses mainly on the combat. On that note, I tried out all the classes.
I felt like the Soldier really hasn't changed a whole lot, which is actually a good thing. They're still the best with weapons, and Adrenaline Rush is still a handy tool, if not quite as strong as it was in Mass Effect 2. Ammo Powers are a little bit weaker, but the guns feel stronger, so it kinda balances out.
Their Tactical Cloak has been improved so flipping much. It can even be improved to let you launch another power and REMAIN cloaked. While the demo didn't give you a Sniper Rifle, Infiltrators feel more comfortable in close-quarter combat. The new Sabotage skill really helps too.
With the major improvements made to melee combat, Vanguards are POWERHOUSES. I used my gun more for bashing faces than shooting, and the way Charge can be improved truly makes Vanguards kings/queens of mobility. Not to mention, their heavy attack is quite useful at clearing some space, and it feels good to Falcon Punch things. Shotguns, the main weapon I use as a Vanguard, also seem more powerful.
I feel that they've gotten weaker, though its mostly because the Demo isn't all that good on "caster" classes. And yes, I think of the Sentinel as a caster class, not a heavy tank class. Their Lift Grenade power is useful though, as you can finally Warp-detonate by yourself. However, their Tech Armor really messes with their cool downs. A useful trick, however, is that in the demo I could re-map my Y button, freeing up another slot for active powers while using Tech Armor from the power menu.
Since I started playing Multi-Player, I've actually come to a different conclusion about Sentinels, since I can build them up to be chain casters. Tech Armor doesn't nearly mess around with cool downs as much when running with a single weapon, and the ability to Biotic Detonate with Throw is something that shouldn't be looked down upon. They've once again become my favorite class with good, all-around support.
I was never one for Adepts, but they feel so much more powerful than their Mass Effect 2 version. No where near the god-like powers of their Mass Effect 1 selves, but a good step in that direction. Unfortunately, all the weapons you're given in the Demo really weigh you down and mess with your cool downs something fierce. So if you want to make fierce use of your powers, like an Adept should, you'll be severely gimped on your choice of weapons. On Multi-Player, I've found their impressive crowd control makes them godly in a support role.
Holy crap. I don't think any class has been improved as much throughout its Mass Effect career as the Engineer. First off, you have all the benefits of Mass Effect 2, with two major changes. AI Hacking is semi-gone, being condensed into Sabotage, giving Engineers more control. Second, Sentry Turret. Even on the Demo, with all the weapons, both Sentry Turret and Combat Drone (which has a shield now!) can be out at once thanks to their long-life spam. That ties up so many enemies. Also, Overload has been improved, now doing greater damage to Barriers (which there weren't any to truly test on,) and can have Neural Shock built into it. Give Engineers as few weapons as you can, like an Adapt, and you'll have a chain-caster that cannot be matched.