Overall EP I got the most bash by the fans since people had very high expectation of it, I do like it since it's the first SW movie I've watched but I can see the problem.

The beginning of EP I is not simply good and evil conflict, Trade Federation's blockade of Naboo is just a step of Plagueis and Sidious' great conspiracy. So unlike EP IV the subtitles, Vader and the Executioner flagship are enough to let people understand the conflict well. Without looking at the EU works, it's indeed a bit hard to understand the situation. The movie also didn't explain well enough about why the Trade Federation would obey the mysterious Sidious. Before people could understand what's going on, the Jedi started to fight the droids. Darth Maul got nearly no characterization in EP I, he was just used to let we see a lightsaber duel. Sure Boba also got little characterization in OT but their role in the movie are a bit different. The 14 years old Naboo Queen has been bashed a lot by the fans as well. In addition, Jar Jar proved Lucas were not good at create comic relief characters.

I think there are 2 major problems of EP I, one is the movie didn't get the age of the audience right. EP I generally is the closest to a “kids movie” of the Six, a 9-years-old child as a protagonist, 14-year-old queen, Jar Jar, the Gungans played an important role in the war. and the battle was ended by the kid Anakin. I don't know did Lucas did this on purpose, but these really made EP I like a movie for kids. I went to watch it when I was a kid and I like it, when I went to watch the 3D last year I clearly feel it's a bit childish. Most of the SW fans were at least over 20 years old or even 40-50 years old in 1999, they might be kids during OT but certainly not any more in 1999. This is a important reason why people bash EP I so much.

Another problem, also one of the biggest problems of the entire PT is Lucas wanted to tell a more complexed theme in PT, the theme is actually better than PT. But he was not able to present the theme and the story well in 3 movies of 2 hours each, and since there were too much to tell, the story and characters were not focused enough. Despite various EU did explain the story, most of the fans will not go to read these things. Of course, a good movie indeed should not rely on other sources to make the audience understand. Naboo crisis and Maul's lack of characterization were both caused by it.

Of course, EP I has quite a few highlights. Liam Neeson showed us a great, passionate and even a bit rebellious Jedi Master. When I watched EP I I thought "that's what a Jedi Master should look and act like". He also behaved like a father figure to Anakin. Anakin and Obi-Wan were not very close to each other, Obi Wan even supported the council's decision, we could see why the two parted ways later. It revealed the stubbornness of the Old Jedi well. Palpatine also showed up as a handsome, elegant but also ambitious and insidious politician rather than the "symbol of evil" Emperor we saw in OT.

Overall, EP I clearly showed prequel trilogy had great theme and more complexed conflict, but Lucas wasn't able to present so much stuff in the movies well, many things were "skimming over the surface", lack of focus on the story and some of the characters, and the mess up with the audience's age. Although there are many highlights but I understand why the OT fans hate it so much. Of course, after reading the EU of this era, we could understand the whole blockade and crisis of Naboo well, while we still have to endure some childish plots and elements, like the Ewoks in Endor of EP VI.