Tbh I think Xul really needs some work - he's the only hero I've bought on release because the necro was my favourite D2 class, but he doesn't feel up to scratch right now (having both played him for a couple hours, then spent another hour or two playing other heroes against him). Maybe they decided to play it safe after the absurdity of LM? The basic concept feels solid, but some tweaking is definitely in order to make him feel like a viable choice and give him a defined role.
Right now it feels like they intended for him to have a bigger lane presence, but the skeletons are so weak that just isn't happening - his W gives him good clearing, but a shitload of heroes can clear just as well. In team fights his role basically boils down to running in with his shield up, picking a target for his E, dumping his cooldowns, then trying to run back out without dying and spending the next 25 seconds picking off stragglers around the edges until his shield is back up.
Most obviously, skeles are just shit - they don't do anything against enemy heroes, can't be controlled in any way and have the hit pool of wet paper towels. HP should easily be 2 - 3x what it is now or they might as well not exist - it's hard to say whether the damage is adequate because they die so quickly there's no chance to gauge it. They just don't add anything to the hero in practical terms, you could remove them and the playstyle would barely change (although I liked using that talent which gives you HP every time one spawns to top up my health on lane mobs between fights).
Trying to fit in a control might be a bit clunky on top of Bone Shield, but they could at least give a way to spawn them outside of lanes and participate in hero fights - maybe attach them to Bone Shield, so when it's cast, or wears off, some skeletons spawn and attack the nearest enemy, or your most recent target? Or a talent that spawns a certain number of skeletons whenever the necro uses a certain ability/abilities (I know they have this with the root, but 2 skeles for 1 spell isn't enough, and you have to give up the extra range on Q).
Bone Shield itself also feels underwhelming and a bit fiddly - my first impression is that it's not really enough for the main survivability tool for an otherwise squishy and non-mobile melee character, both in terms of duration (3 seconds out of 30, or 40 if you're against an AA heavy setup and go for the #3 talent) and in terms of general effectiveness. You have to activate it, which makes it another button to jam when you suddenly get a bunch of DPS on your ass out of nowhere, and it has a tiny duration, so as soon as it's down you find yourself having to play a lot more conservatively, which just doesn't work when you're primarily a melee character.
My first thought was that they should get rid of the duration limit and make it permanent (ie. you cast it, get the shield, then when it's knocked off you can cast it again after the cooldown duration runs out). This would help especially with the whole issue of getting jumped. Could also fiddle around the duration, cooldown, percentage, HP pool etc.
Q is ok damage wise, but way too fucking easy to dodge for anyone who's actually watching the screen, unless you got them with E, in which case the point is moot. I'm thinking maybe they could remove the 1 second delay before it goes off. Or at least make it move faster.
W seems solid, although it would be cool if they branched out with the "Curse" theme via talents - they could use this to help specialize him more, maybe givingsomething like Sylv's building shutdown to help with laning (or make the attack speed debuff apply to buildings), or something like a weaker version of the reduced healing from his lvl 20 #3 for team fights, and so forth.
E feels a little off. I get that they didn't want to make it too OP, being what it is, but the really tiny range and the long delay combined with lack of closing abilities makes it a pain to use as an opener, they need to give it some leeway with one or the other unless they intend to make him capable of staying in the thick of things a bit longer, imo.
Talent wise he's also lacking a fair bit - a lot of the the choices feel really weak, there's more or less no diversity in specs and even before the usual websites had recommended builds up, everyone was speccing 90% identical because almost every tier has a particular talent which stands out as well above the others (or at least, not as bland as the others), and even where there are choices they don't actually make a huge impact on the playstyle. They could really spruce things up here to make some of the tiers more exciting and create more diversity, since right now the only real difference I'm noticing is between the people who realize that Skeletal Mages beats the shit out of Poison Nova as an ulti and the people who don't :/
Overall I get the impression that they spent so long fiddling with the character and how they wanted it to play that other things like the talents and build diversity went by the wayside a bit, but the basic concept is solid and with a little work I think it could be an awesome hero (or maybe I'm just being optimistic for nostalgia reasons).