Lightfoot Halfling Rogue
Halfling traits allow you to hide behind creatures larger than you, and to re-roll something twice per day. I find the hiding thing to be a bit silly, but whatever. This Rogue has the Commoner background (backgrounds determine skills) which allows you to choose a common profession, gaining knowledge and contacts relevant to your trade. You also gain the skills Animal Handling, Commerce, and Folklore (as you can see, the skills are very open-ended and almost resemble Traits in The One Ring). Training in a skill gives you a +3 bonus to any relevant attribute check.
Rogue builds are known as "schemes," and this guy is a Thief. This seems to be a heavily skill-based scheme, with less of an emphasis on combat. You can hide while only lightly obscured (or with only a quarter cover), you gain knowledge of Thieves' Cant, and you have training in Open Locks, Find/Remove Traps, and Stealth. Aside from your scheme, a Rogue also has Skill Mastery, which lets you take 10 if you roll lower than that with a trained skill. One bug related to this is that the Rogue pregen is just 1 away from auto-succeeding at finding any trap in the sample adventure. Given that perception is a Wisdom roll, and this Rogue has a Wisdom of 8, that's very problematic. Unless adjusted, most Rogues will earn too many auto-successes. Finally, Rogues get free Thieves Tools and, of course, the ability to Sneak Attack. For the 3 levels provided, Sneak Attack scales by 1D6/level. That could get messy. Additionally, while it's not worth spending your action to Hide just for Sneak Attack at level 1, by level 3 it becomes a more viable tactic. That's my biggest sticking point with this Rogue; a signature ability varies in its viability based on level.
The Rogue's theme is Lurker, which gives him various perks while hiding. Themes are really just pre-selected feats, and the designers talk about them as defining how you do your job. In broad terms, class is your combat abilities (and sometimes a few extras), background gives you skills, and theme is your style (how you use your abilities).
Hill Dwarf Fighter
I've seen a lot of complaints about this Fighter being too simple. The designers assure us that a more tactical 4E-style Fighter is forthcoming, and I don't lament that they decided to test drive the core rules with the simple version. They need to get the simple Fighter balanced and polished before they can design something more complicated, after all.
Hill Dwarves apparently get a boost to their Hit Die size, racial weapon affinities take the form of moving the damage die one step up, and Dwarves also get a list of other nifty racial traits (immunity to poison, low light vision, no speed penalty for being encumbered, and the ability to identify stonework and not get lost underground).
The Fighter's background is Soldier, and this lets him perform strenuous physical activity twice as long as normal. This is things like forced marches, holding your breath, etc. The trained skills that he gets are Intimidate, Perception, and Survival. These are a nice set of skills! I love how base skills are divorced from class now; no more being a second-class skill user just because you're a Fighter.
Simply put, Fighters have good armor, good weapon proficiencies, deal the most damage, and are tough. That's all this simple Fighter is, really, is class-based "better numbers." His theme (Slayer) provides a little more variety, however. The Reaper feat lets him deal Str mod damage even on a miss, and at 3rd level he gets Cleave, which lets you make another melee attack when you kill something. Nice! Oh yeah, and at 2nd level he gets something called Fighter's Surge, which is basically a twice per day action point.
Depending on what kind of guidance DMs are given on how to adjudicate improvised actions even the simple Fighter has the potential to be fairly interesting. Specifically, I don't think it would be a stretch to allow players to tack improvised actions onto their attacks (push enemies back, knock them prone, etc.) to "create" 4E style powers on the fly. Possible drawbacks might include a penalty to the attack, attacking with disadvantage, granting advantage to your opponents, or sacrificing some of your damage.
Human Cleric of Pelor
Humans appear to get bonuses in all of their attributes, and not much else. Interesting. This guy's background is priest, and that gives him free services when in a temple to his god. He also gains Diplomacy, Insight, Religious Lore, and Wilderness Lore as trained skills.
Clerics are kind of what you'd expect; they get a decent Hit Die, can wear good armor, get decent weapon selection, and they cast spells. They seem to get fewer spells than the Wizard, but can cast from a list of chosen spells spontaneously!!! I really hope that the Druid gets to do this as well. This Cleric gets Radiant Lance as his at-will spell (basically a big laser), as well as Detect Magic. He can also cast Cure Light Wounds, Spiritual Hammer, and Searing Light (a bigger laser). Clerics can also use Channel Divinity 4x/day, but you only start out with Turn Undead. You can channel other stuff as you level.
This pregen has the Healer theme, and it really turns him into a healbot. Your start with the Herbalism feat, which is basically "make antitoxin, healing potions, and healing kits for half price!" I can't say that I like this, as it hearkens back to the "spend gold for healing!" logic of 3rd edition (can we say "Wand of Cure Light Wounds?"). At 3rd level you maximize healing dice. Sick.
Mountain Dwarf Cleric of Moradin
Same traits as the Hill Dwarf, except I'm presuming the stat bonus is different. Also, it appears to get +1 to AC instead of a Hit Die increase, and its racial affinity is for hammers, not axes. Ok, cool.
This guy's background is Knight (can we say "Paladin?"), which gives you social perks when around nobility or others who recognize your station. You also gain training in Animal Handling, Diplomacy, Heraldic Lore, and Religious Lore.
The reason why 2 Cleric pregens were chosen was to highlight how your god can alter the playstyle of the class. Indeed! Whereas the Cleric of Pelor is a "zappy, zappy laser, oh you're hurt here's a heal spellcaster" this guy's very much an in-your-face warrior who buffs himself with his divine magic. His only at-will is Death Ward (which lets you stabilize your dying friends), and his prepared spells are Crusader's Strike (my hammer does more damage!), Divine Favor (buff those attacks!), and Healing Word (I'll heal you just a little bit as I'm smacking someone in the face). Again, he can choose to cast any of these spontaneously (but only 2x/day).
This PC has the most interesting Theme of the bunch, since as a Guardian he's kind of a defender. At 1st level you can use your reaction to give an attacker disadvantage when they attack an adjacent ally (you partially block it with your shield). At third level you say "you shall not pass!" to creatures your size or smaller, and they stop their movement when they enter your reach. Still not quite as functional as a 4E defender's mark + punishment, so we'll see how this develops.
High Elf Wizard
I'm sure the Elf gets some "hidden" stuff not directly stated on the character sheet (like the die increases of the Dwarves), but I don't know what they are. They do get their immunity to charm and sleep, auto-advantage to listen/search/notice, and low light vision.
The Sage background is thematic, and its Researcher trait allows you to know where to find a piece of information even if you fail a Lore check. Trained skills are Forbidden Lore, Magical Lore, Natural Lore, and Religious Lore. Forbidden Lore sure looks interesting (if situational)!
So casting restrictions are back. As in, you can't cast spells while wearing any type of armor, and when you take damage you have to make a Con check if you try to cast a non at-will spell. If you fail the action is wasted (but the spell isn't used). Wizards also have to prepare the exact spell that they want to use from their spellbook (didn't prepare Burning Hands, can't use it). Yeah, I like the Cleric's magic better. This Wizard does get a lot of spells, though. And his prepared spells do seem to be a good deal better than the Cleric's. Burning Hands, Comprehend Languages, Shield, and Sleep are your 1st level spells, and your at-wills (minor spells) are Detect Magic, Light, Mage Hand, Magic Missile, Ray of Frost, and Shocking Grasp. Ray of Frost does no damage but reduces speed to 0, which is interesting (you need to make a ranged attack to hit - oh, and a ranged attack with magic means you use your Int vs AC, so no need to have a decent Dex score just to hit stuff).
Two of those minor spells come from the Wizard's theme, Magic User. At 3rd level you get a familiar (which you can use touch spells with, hello Shocking Grasp!). Familiars are more like their 4E incarnations, in that they don't die permanently and you don't have to worry about Con loss.