Many of the MCU films (including the previous Avengers movie) were criticized for having bland or underwhelming villains. Kevin Feige has acknowledged this criticism, and has said the movie will give a great deal of focus to Thanos in order to flesh out his motivations and show the audience why he's such a threat. He went so far as to say that in a way, Thanos could be seen as the main character of Infinity War.
The movie fixes the continuity issue that was caused by Spider-Man: Homecoming, with Tony mentioning it's been 6 years since The Avengers, falling in line with the 2012 date given by other MCU films and series.
For those fans who felt that Captain America: Civil War glossed over the possible negative repercussions putting the government in direct control of the Avengers could have, the film includes a scene where Secretary Ross attempts to use the Sokovia Accords to make Rhodey arrest the fugitive Avengers at the expense of investigating and combatting Earth's latest alien threat, making Rhodey realize signing them as written probably wasn't the best idea.
One of the biggest complaints about Doctor Strange is that for the most part the fighting was done with punching and generated weapons like swords and shields, which made the fight scenes functionally no different than the other movies, even if it was justified by Strange's inexperience. Here Strange's fights utilize much more spells and his fight with Thanos is a full-scale, reality-bending Wizard Duel.
The continued downplaying of the romance between Bruce and Natasha, with them being a bit awkward at their first re-meeting but with no other reference to it.
The film seems to have taken advantage of criticisms about Dr. Strange being extremely similar to Tony Stark by giving them good character interaction, in the form of their insufferable personalities clashing with each other.
Those who felt that Peter's Stark-made suits gave him too many convenient powers in Spider-Man: Homecoming may be pleased that, despite getting the Iron Spider suit, Peter's webslinging powers are still at the forefront. In fact, the only extra ability here is the mechanical spider legs that only show up about three times.