Being still fairly new to HotS I've been trying out different heroes the last few days.
Valla - Flippin' awesome. Easy damage. Does she have any glaring weaknesses?
Not really - she has lower health than a lot of heroes (Valla at 20 has less health than Tychus at 10), but she pretty much does everything. Can be a tank killer, can be a duellist, can be a backline poker. Very hard to find a team she doens't fit in.
Raynor - Boring! Also easy damage and has some toughness with the regen but he's got too many passive abilities for me to really enjoy.
Yep - deliberately designed to be really simple and easy to play as your first ever hero. Problem at the moment is his Hyperion Ultimate is bugged and wildly overpowered for team fights, which means you have to play him or ban him.
Sonya - Talked about her a little bit already but she seems to be a little bit of a warrior/assassin hybrid, doesn't she? Sick burst damage for a warrior but lacks escape and feels a little bit squishy overall.
Yes, she's a bruiser not a warrior, and should be played like that - you need a proper tank to go with her. Used to be OP, now a bit lack lustre compare to other melee assassins. She does still have the issue that at 20 she becomes practically unkillable which is annoying. Her leap build is very good at deleting backlines, esp in conjunction with other AE effects.
Xul - Surprisingly fun. Reliable root. Melee AoE. Skeleton minions. Does he have a role in the current meta?
In standard hero league, heroes with great wave clear and pushing power like xul are always going to be decent I feel. He's particulary good on some maps (Blackhearts Bat, Infernal Shrines, Tomb of the spider queen) when he can rotate between two lanes and push both, but honestly he's never really a bad pick in a standard team as long as you also have ranged damage to back his roots up.
The Butcher - Late game with 125 meat he's a giggle a minute. Absolutely devastating damage. Having only played him vs AI I can imagine that he might have issues staying alive against a competent team that knows to make him a priority. Seems like he might be a force to be reckoned with but might need a babysitter to be fully viable. Still, he's really fun.
No idea tbh. Has a really high win rate but I don't see it at all, as you can just blow him up when he goes in in my experience. Very snowbally hero, who is the opposite of what he used ot be (at launch was an early game gank machine, now he needs to farm his stacks and comes online once he completes that quest). I'd say there were strong melee assasins to focus on - Thrall, Alarak, Varian, Artanis in particular.
On a related note, who are the basic "must have" characters that should be unlocked ASAP? I just want to be sure I have my bases covered as I go forward and make sure I dont have any glaring holes if I want to actually play against human beings once in a while.
You ideally would want a few heroes in each role. even if you don't intend on playing the role at some point you'll end up in it so it's good to have at least one fallback. My suggestions fro someone new to the game would be:
Tank: Muradin, ETC, then Diablo. Muradin and ETC are staples, both do the tank job well, both set up kills and disrupt the enemy team. Diablo is also fantastic BUT he comes with a big caveat - it's easy to play him poorly, and he can often fuck up your own team by charging or flipping people inappropriatly. Play him by all means but watch out for that!
Ranged DPS: Valla, Tychus, Jaina. Ignoring the bugged Raynor issue, those three give you a versatile sustained dps, a tank killer and the best burst mage currently available. There's a huge selection to pick from tbh, and arguments can be made for many many heroe,s but I'm never upset to see those three in my draft.
(Tychus in particular IS the meta-game right now outside of Raynors bugs. I don't think most people appreciate how he affects drafts). All three have multiple viable builds as well which I think important for game enjoyment.
Specialists: Xul, Sylvanas. Those are the two you might be aksed to play the most, both allow for split pushing and ultra fast wave clear. On some maps if you don't have Sylvanas it can really fuck your teams draft (Battlefield, Braxxis, Haunted Mines, Infernal Shrines).
Melee DPS: Thrall, Thrall, Thrall. Until earthquake is nerfed he's the best melee assassin by far. After that Ragnaros I'd suggest.
Support: Rehgar, Malfurion, Brightwing. Auriel as well if you like the role, but those are the main three you should have in your pocket.
Note there are LOTS of other heroes you can go with, and several who are better than the above. All that list is an attempt to sum up the common "really common picks" that you will either play with or against in a lot of games, so you need to know them and how they work. I'm sure lots of people will point out there are better choices in all the tiers, but honestly think that's a decent starting point for someone new the game.