Lv1: Extra distance allows you to reach further into the back line, and also allows you to improve your gank potential. You do have to change your playstyle a bit considering you can't Q into E anymore because you shove them too far away. (Unless they hit a wall)

Lv4: Body blocking is a HUGE part of your bread and butter as Diablo. When you overpower someone or engage, pop your fire stomp, then stick to them preventing them from going where they want. This gets you a bit more damage and allows you to not only clear lane better, but also hurt the enemy even more when your abilities are on cooldown.

Lv7: Battle momentum is a no brainer. When you're body blocking you're also mixing in auto attacks, allowing you to use your CC even more often.

Lv10: This is actually up for debate. If you need the damage, go Lightning Breath. I personally perfer Apocalypse because it's even more CC and I usually have enough damage in my team comp.

Lv13: I used to go firestorm, but considering the nerf that just happened, it opens up the less used options of spell shield and siphon of the dead. Go spell shield if you're up against a lot of ability heavy characters, and otherwise I'd pick siphon for a little bit more sustain. You can still pick firestorm if you want damage, just keep your options open.

lv16: Domination is for me, the best choice. Your Q is probably the best ability in your kit and being able to use it more often is your highest priority. Charge in, overpower, body block for moment, then your Q will be right back up to shove someone else to their death.

Lv20: This level is very up in the air. You can either get the upgrade for your ult, bolt of the storm, OR Lord of Terror. I'm not trying to tank, so I stay away from Lord of Terror; and an upgrade on apoc isn't the best for me either. This leaves me with one choice, Bolt. An odd choice indeed, but being able to blink over walls to blindside the enemy or to chase or disengage is too strong an option for one of the best gankers in the game.

Diablo USED to be of of the best front liners of Heroes of the Storm, and you could build him to have enormous amounts of HP. Now, times have changed and we have to adapt. Personally, I always saw him as more of a CC train than a tank, but that's just me.

If you want to front line, do not pick Diablo. Simple as that. Front liners need 2 things, a large HP pool, and the ability to die and come back whatever time later just as effective. If you put talents that rely on Diablo's passive, your effectiveness will be in flux the entire game. (Unless it's a stomp you will probably die a few times) Use his passive to create openings for your team and allow your teammates to take advantage of your CC without having to worry about you as much. You can front line in a pinch, but he's not the best at it. Duo warrior comps are fantastic for Diablo, with the addition of some burst damage to make the best of the openings you make. Not only burst of course, a good balance of burst and sustained damage is always important.

Your passive should really be saved late game, as it allows you to dive in to create openings, cause a few enemy deaths, and then be ready to go a few seconds later for the ensuing push. (Asuming it's late game and you won the fight) Sacrifice yourself for the betterment of your team but don't play stupid. It's a fine line, but the more of Diablo you play the better of a feel you'll be able to get for it.

You have a few different options when you engage.
-You open with your E, go into fire stomp bodyblock then as your team pushes forwards Q even deeper in disrupting the enemy line or use your Q to grab someone and disengage.

-You OPEN with your Q, possibly shoving their tank out of the front line (possibly popping Apocalypse as soon as you can cause odds are they're gonna try to collapse on you.), walk forwards grabbing someone with your E, then popping your fire stomp and body blocking.

-The last option is opening with your ult for superb disruption, possibly catching a few enemies out. Position well and you can shove them into the waiting hands of your team, use your E on someone and you can shove somoene else to their death late game too!

-The addition of blink allows you to do even more, although the skill cap raises massively. You blink behind and Q as quickly as you can, fire stomp, body block, then overpower if they're still alive, then (if you got the talent) shadow charge again. It's hard and VERY risy, thus I mainly chose to use blink defensively, or to position better. But all the same, that's why I like blink. It has so many uses that it makes you supremely flexible.

Really though, use your abilities in whatever order works best for YOU. These are just suggestions I'm giving to try to get you familiar with Diablo as a hero.