Having been in the beta for several days now and completing it on 3 of the 4 classes on 2 of the difficulties I have to say that Torchlight 2 is one of the funnest action rpgs and one of the funnest games I've played in a while.
As far as isometric action rpgs go I have only ever played the original Torchlight, Titan Quest, Bastion and a bit of time in the Path of Exile beta and I've put more time in Torchlight than the others. Comparing Torchlight 2 to the original I would say that Torchlight 2 upon Torchlight 1 in every way.
The gameplay in Torchlight 2 is faster than the first game and the combat feels crunchier. Basic attacks in Torchlight 2 have a good amount of impact and the skills feel really satisfying to use. Every weapon type works differently than the others in some way and adds a dimension to making your playstyle as you have to decide whether or not you prefer the mid-range arc of a shotgonne to the long-range shots of a crossbow or whether you prefer the quick armor-ignoring strikes of claws to the slow wide arcs of a hammer. In Torchlight 2 you can equip any non class-specific piece of gear as long as you reach either a level requirement or certain statistical requirements.
Like in Torchlight 1 there are 4 stats in Torchlight 2: Strength, Focus, Dexterity and Vitality. Unlike Torchlight 1 every stat is useful for every class in number 2. Strength increases your attack and critical damage, Dexterity increases your dodge and critical hit chance, Focus increases your maximum mana points, increases your magic damage and also increases your chance to execute which is when you strike with both weapons at once if you are dual-wielding 2 of the same weapon type. Vitality increases your health pool and your armor rating.
The classes in Torchlight 2 have a good deal more uniqueness to them than the ones in the first game did. They each have their own passive charge bar which affects their playstyle in some way. They also have a lot more active skills compared to Torchlight 1 and what passive skills they do have are unique to the classes and aren't shared between them. There are a few balance issues with a few of the skills and certain builds in the beta, and the skill trees are also going to be changing in some ways before launch but the changes won't be in the beta so I can't comment on that.
The 4 classes in Torchlight 2 consists of the Berserker, the Outlander, the Embermage and the Engineer.
The Berserker is my favorite class of the 4. The Berserker focuses more on dual-wielding and getting critical hits, although using 2-handed weapons is perfectly viable. The Berserker's charge bar fills up every time you hit an enemy and when it's full you enter a Frenzy state in which you gain increased movement speed and all of your attacks are critical hits. The Berserker can put points into either the Hunter, Tundra or Shadow trees. The hunter tree is focused more on melee skills and hitting hard. The tundra tree is focused more on ice damage and magical type attacks. The shadow tree is focused more on bypassing armor, hitting groups of enemies and life-leeching.
The Engineer is more of a tankey type class than any of the others. The Engineer's charge bar consists of 5 pips which fill up when you attack enemies. Certain skills use the pips on the charge bar to become more powerful. The Engineer can put points into either the Blitz, Construction or Aegis trees. The Blitz tree focuses more on using 2-handed melee weapons and fighting groups of enemies. The Construction tree focuses on using mechanical-eque skills like summoning a turret or heal bot or lobbing a grenade. The Aegis tree focuses on using a shield in combat and protecting your allies by tanking blows.
The Embermage is your standard mage class. The Embermage's charge bar is rather simple, when it's full your magic attacks are 25% more powerful and consume no mana. The Embermage can put points into either the Inferno, Frost or Storm trees. The Inferno tree is focused on hard-hitting fire attacks, whether they hit a large area or only a single target. The Frost tree isn't as damage focused as the Inferno tree but puts more emphasis on crowd-control with slowing and immobilizing skills. The Storm tree is based around short-range electric damage and causing knockback and explosions on death.
I can't speak very much on the Outlander as I have only played it once and I haven't played it all the way through the beta. The Outlander's charge bar gives them a variety of stat boosts as it fills up. They get improved cast speed, chance to dodge and get critical hits and improved attack speed. They can put points into either their Warfare, Lore or Sigil trees. The Warfare tree is focused on using ranged weapons such as bows, crossbows, shotgonnes and pistols. The Lore tree is focused on keeping enemies away by using the Outlander's glaive. The Sigil tree seems to be focused around using curse type attacks on your enemies.
The world of Torchlight 2 is also done much better than in Torchlight 1. Whereas in Torchlight 1 you only had a single town and 1 long dungeon that got a bit samey after a while, the beta for Torchlight 2 had a single town with several over-world areas, each with at least 1 dungeon in them. There were 2 rather large and open main areas that had several dungeons each in them. These areas also had several different kinds of monsters in different areas that helped make the areas actually feel like a world. There were also a couple of "transition" areas that connected the larger areas to the town. These were smaller and more linear than the other zones and only had 1 dungeon each in them. The 2 larger zones also had waypoints near the entrances that connected to the town so you wouldn't have to run through the transition zones constantly to get back to town. The areas are also randomized much better than in Torchlight 1. Unlike in Torchlight 1 where the areas started to feel samey, areas in Torchlight 2 are randomized with many more set-pieces and in a much more varied way so the same areas don't always seem like the same places.
I've played both by myself and in groups of random people and the game is fun in both single and multiplayer. Playing singleplayer allows you to take your time more and analyze things better whereas playing in a group of random strangers, while still extremely fun and more challenging due to the scaling of monsters, felt just a bit frantic and like I was constantly rushing everywhere. I'm sure if you were playing with a group of friends or family the pacing will probably be different.
The difficulty of Torchlight 2 is higher than the original game and playing with other people makes the monsters scale up in damage and health and the game becomes even more difficult. I haven't been able to do any multiplayer on Veteran difficulty but in Normal mode some bosses were able to nearly one-shot some of my characters and it made have to actually be careful.
As far as the length of the game goes it has been extremely varied for me. The beta consists of the full Act 1 of the game and when going everywhere and seeing everything I could on 4 characters I have found that it has taken me sometimes just under 6 hours on some characters and sometimes just under 12 on others. Acts 2 and 3 are supposed to be even bigger than the first and there is supposed to be a fourth act after that.
All-in-all I would say that Torchlight 2 is one of the funnest action rpgs I've ever played and is already well worth the $20 that I'll be spending on it.