Whedon's true mistake: Overestimating his audience.

Rating: 9/10 (7 May 2015)

Prior to watching the movie this is what I expected based on most review going on at the moment: Full of action, bad storytelling, something weird with Black Widow, and that Ultron was an unsatisfying villain. After watching the movie I can now add that most of what people are complaining about, why people are giving the movie very low scores, is highly over dramatized.
I would say to enjoy Age of Ultron at the level I did you must fill two prerequisites prior to seeing the movie. The first is that you have a decent foundation of the other Marvel MCU movies. You must have some knowledge of what went on in Iron Man 1, the first Avengers, the Winter Soldier, probably Thor 1, and possibly Guardians of the Galaxy. The movie is very quickly paced which I think left lots of viewers feeling lost, knowing these things before hand makes comprehending the plot much easier.
Second, you must be able to fill in the gaps. The movie packs on a ton of characters and weaves together many time-lines, it doesn't have time to explain Black Widow's infamous romance in the movie. It's true it hadn't been talked about before this movie but the writers presumed the audience would figure out that time had passed between Avengers 1 & 2 and actually they offer some lines that make it seem highly plausible. BUT YOU GOT TO PAY ATTENTION!
Other things they complained about: Ultron's inception, explained in plenty of detail. His plan, again explained in detail. Character motives, either alludes to previous movies or are explained.
Bottom line is the critics are mad because they don't understand, and they don't understand because they want to be spoon-fed all this information. Whedon is a clever writer who provided many lines and subplots that should have allowed viewers to understand the movie in better detail. In order to have done what critics wanted, the movie would have to have been much longer and it would have felt drawn out, and they would have hated it for that fact too.
Also another thing about this movie is that it is supposed to serve as a bridge to "Phase 3" of the MCU (Marvel Cinematic Universe). What happens is it is supposed to leave open endings for the characters who will have later movies, to give them a direction to head in, to ultimately lead towards Infinity Wars. This is roughly the equivalent of the Harry Potter movies leading up to the war against Voldemort. A lot needs to be said and done before it can be made, Age of Ultron definitely sends them off in the direction they need to be.
It's either you catch the small things that were said to make sense of the movie, or you think it is all nonsensical. This turns out to be one that I'm confident critics are wrong about. Go to the movie, pay attention, enjoy the amazing action sequences, and understand its flaws resulted from necessity. 9/10 to combat the hate! :)