Your progression can be destroyed if you play with friends who are higher level.

I joined my friend's party while I was still in Act 1 and found myself viewing the Act 4 cutscene, pop up in the Act 4 hub town and was even able to join the fight of an Act 4 boss (and was killed in one hit)

That's called boosting and it has been around since D2. People boosting level 1s in Hell Baal runs were pretty common.

I use the Passive skill that grants +60% movement after vaulting and have the stun and knockback on it as well as a rune to kick away minions when I escape. Belial is also a challenge for a couple of slower classes since he basically carpet bombs what is a very small arena with his AoE spells. Azmodan was, to me, just way too simple, I can't imagine having any trouble with him on any difficulty unless he unlocks some new incredible skill.

Diablo's lightning/magic/fire attack from Diablo II is back but the reach has been almost doubled it seems! That could certainly be a pain for slower classes and since he has so much HP, I can imagine that he must be pretty hard! His immunity to slow doesn't help either to be honest. I've also noticed that the boss groups (regular mobs) and elites have grown rather fierce on Nightmare, which is a good thing but the game is still very far from challenging as a decently equipped Demon Hunter.

Byt the way, does anyone use any of the followers? I use the Enchantress, she does almost no damage at all but she soaks some (and followers never truly die) and her pushback attack is extremely handy when facing large groups of mobs.

I think you'd find it a tad difficult to beat level 25-30 enemies in the beginning!

Diablo has always been like this, and this is common in other genres as well, no?

Belial was pretty easy for me, because even though he carpet bombs the arena, there's always space to run to. Azmodan on the other hand, fills the arena with that leech AoE and that can take a slow class by surprise. 

Diablo wouldn't be too hard for a Witch Doctor if he didn't aggro on you even though there's other 6 minions pelting at him . The second phase of the battle is the hardest one, since it's pretty hard to discern the shadows from the bone prison and it's usually too late to run from them if you don't do it right from the start. 

Elite mobs on Nightmare can be challenging depending on the mods they have. I dread to think if there's a Mallet Lord mob with Vortex and Prison on Act IV Nightmare >_<.

I use the Templar because he aggroes enemies a lot and has a lot of durability as well (plus his A.I for Heal is rather good).

Just finished Act I on Nightmare. Pretty simple but I expected nothing more, Act II will be a lot harder for sure, the movs are a lot better here. But, then again, so are the drops...

Up to 1250 DPS now!

add me, bugrimmar the barbarian :D

I've got to say this: Demon Hunter with Evasive Shot as primary, Rapid Fire (withering fire rune) as Secondary and Tactical Advantage as passive = extremely OP XD

Whats with the spoilers guys?

Yeah, watch out for spoilers everybody.

I'm still surprised at how much you've all played so far. I thought I played quite a bit and I'm at the beginning of Act 3 Normal.