Ultron minions == Chitauri in the sense that even a human such as Black Widow or Hawkeye can take them down.

Ultron himself - well, I don't have high hopes for him to be able to kill even  1 Avenger. For all his talk about how mankind needs to be eradicated, I doubt he'll be able to do much damage.

We already know he's probably not going to kill an Avenger because of the casts for the next Marvel films. As for him looking weak, I don't think he needs to kill someone to solidify his strength, but we'll have to wait and see.

Black Widow and Hawkeye can just have weapons built for electronics like Ultron bots to be taken down. He doesn't have to kill anyone to be strong he just needs to knock down Thor or Hulk or maybe tear apart an Iron Man Armor. Hulk better not beat Hulkbuster unless Tony says something before hand "I can only hold him for a couple minutes" but from the trailers it looks like he loses and Black Widow calms Hulk down. Stark shouldn't be surprised by Hulk's strength though.

Its a machine! Get an electro magnetic pulse and you can be a hero aswell.

This is fantasy you know... maybe let go of the details and enjoy.

he needs some more robot roids!

Ultron starts off weak and possibly even a servant. He gets smarter and stronger. He eventually becomes indestructible (he is covered with the same metal as Wolverine's claws). Not only that, if they stick to the comics, he will be able to make weaker copies of himself (not covered in Adimantium), control electronics (like Iron Man's armor--while he's wearing it!), and control minds.

Ultron is often depicted as the harbinger of armageddon in many stories. He ain't no scrub. He'll be up to the task.

From the looks of the trailer, he broke Captain America's shield. As far as I know ,that shield is supposed to be indestructible...

Oh, he also once blew up a country.

Yeah his minions do look pretty weak, I prefer something that can be feared and his minions certainly can't the rate they take them down in the trailer. Like the Agents from The Matrix. First movie they're godlike, next movies they're much much weaker....

Though i'm sure Ultron himself will be pretty beastly.

I assume (and I have never read a comic in my life) if left unchecked, Ultron can develop and "evolve" to be a real mother fucker. I don't know the time span that happens from beginning to end of the movie, I assume maybe a few weeks/months.