The Good:
Dialog-
All great, with solid banter between the major characters. Spader was solid as Ultron, despite what haters say. The chemistry from the core cast was still there from the first film.
Hawkeye-
Could not be bothered to give two shits about him in the first film, but here he was very well fleshed out and relatable. He was probably the best written character in the movie on all fronts.
Effects-
Second to none. Hulkbuster fight was one of the best sequences in any MCU movie.
Pacing-
Movie did not drag at all, with one exception (see below). It flew right by, which is the sign of a great movie.
Acting-
All top notch, even in the problematic scenes. This is why MCU movies (and First Class) movies succeed where other Super Hero movies fail. Great acting does so much for suspension of disbelief and selling it. If RDJ is truly done, this series will survive, but they can't lose him, Evans, Ruffalo, and Hemsworth if they expect to continue. At least three of those guys need to return.
The Bad:
Quicksilver-
Not really Whedon's fault, but after Days of Future Past, no one was going to make a better version of that character between the action and personality. If DOFP did not exist, he would have been ok, but I could not help but compare the two. I am actually glad they killed him, because I think it will help the franchise in the long run to avoid the comparisons.
Bloated Cast-
I get that these movies are sort of the nexus for the MCU franchise, but so many of the characters felt like they were there just to be there. Samuel L and Peter Skarsgard in particular felt woefully underutilized in a movie that just crammed so many new characters into the mix. I feel like a major reason that Ultron got a little drowned out in the movie was due not having enough screen time with all the other faces they were packing in. There needed to be more dialog exchanges with him and Vision to flesh him out more. Again, this is an area where Xmen-DOFP did a far superior job and they actually had more characters to deal with.
Rehashing the first Movie-
Seriously, this plot was basically a lot like the first movie. Team does not trust each other. Villain exploits this for reasons to buy time to make world ending device. Hulk goes out of control to derail plans. Stark nearly dies to stop device. I think the movie needed to be more original in its story structure. How to do this could be the basis for an entire thread, but lets just start by ditching the entire team rift plot device because the MCU movies went to the well way too many times with that trope.
The Ugly:
Romance-
Someone else pointed it out, but fuck Whedon for bowing down to the SJW Blue hairs and tacking this shit in. It created numerous problems. First, every minute spent talking about Widow's non functioning uterus was one minute lost of Banner/Stark banter or Thor/Vision interaction. It also exposed ScarJo as the weakest member of the cast, again. It did not help at all that I knew why Whedon crammed it in and it pissed me off while watching it. Less romance, more bromance please. The only reason to have a Widow romance on screen at all is to see ScarJo naked, which was never going to happen. If you cut all of this shit out of the movie, it instantly jumps a spot on my list.