Having zapped and fried my way through Caldeum, it was on to the Zoltun Kulle quests and Bastion Keep. I don't know about you, but I found Kulle annoying the first time through. The second time I had to do this, I got him started talking... and left. The Escape key works, too.
Unless you've got really good gear, it's very hard to take down Belial before level 45. I used Archon and Teleport with fractured images, had the Scoundrel out as my companion (highest DPS), and did a lot of dodging and running. I had tried it with Diamond Skin, but that didn't hold up, so I switched to ice armor and teleport. Of course (as is typical), I thought I was relatively indestructible after that and went to the Bastion Keep... where I promptly fell over dead on the first set of monsters.
I'm blaming it on the cold.
Bastion Keep has some good and bad aspects to it. The "good" is that the doorways are narrow, so you can lure the mobs through the doorways in single file. When I switched over to Frost Armor, Ice Hydra, and Frost Nova, things went much better. However, the rule I developed ("if it's blue or golds, switch to Archon IMMEDIATELY.)
At level 45+ in Diablo, it's worth the gold to buy the training manuals and upgrade the jewelcrafter, Covetous Shen. The kind of gear you'll need is horribly expensive on the auction house, but if you keep socketed gears and stuff the sockets with Flawless Square gems you can (sort of) make up for the fact that your pants don't have +60 intelligence on them. Before each big boss fight (Belial, Glutton) you'll want to go through and replace any gear that's 4 levels lower than you. I upgraded the Armorer once after hitting Nightmare level and then discovered that I was using all of my materials plus hundreds of gold pieces just for him to craft something that isn't useful to me. I don't bother putting the blacksmithing pages on the Auction House, either. I just sell them directly.
Things are pretty straightforward until you hit the Battlefields section (after killing the Glutton and running through the lower levels of the Bastion Keep.) Once out in the open, you're a target for everything around. You'll end up dying -- lots. This particular act is always a challenge, and I've seen several recommendations that you tiptoe around and avoid most of the mobs. The Heralds of Pestilence lay down pools of poison that will kill you quickly if you don't move, and they're pretty good about putting those down when you are trapped and can't move.
Perhaps the most surprising thing to me was that here the Templar actually works out better than the Enchantress or the Scoundrel. I had mine set to Intervene, Intimidate, Onslaught, and Guardian (no healing functions) and he picked up the mobs like a proper tank and smote them all... with his very low level damage.