I'm one of those who defend the PT quite strongly, but of course I have complaints. These are two fold:
1. Poor conceptual explanation. GL has lots of good and interesting ideas about the concept of Balance, the Living vs. Cosmic Force, the Prophecy, the problem of attachment and how love casn be a BAD thing, and even the midichlorians... and then promptly doesn't bother explaining any of them on-screen so that you have to check his interviews to know what he was talking about and meanwhile we are left with an understandable amount of fans who therefore read all these things the wrong way, from the "Doesn't balance mean good and evil are the same strength?" crowd to the "He's changed the Force into just a bunch of bacteria!" crowd, neither of which are at all true but you can see why people think it.
2. Too much plot. The original films did well on a three 'planet' formula- Tatooine/DeathStar/Yavin, Hoth/Dagobah/Bespin and Tatooine/Endor/Death Star, with maybe the odd small distraction like the asteroid field or Dagobah re-visit.
TPM immediately ran into some problems by going in a circle- Naboo/Tatooine/Coruscant/Naboo again. The thing that struck me at the time is how basically irrelevant Tatooine was to the wider plot. Its only purpose was to introduce Anakin and I an not sure he HAD to come from there, warching the original films. So anyway, TPM felt a little bit flabby.
AOTC is then the worst offender by far, with Coruscant (given some action at last, good start), Naboo AND Kamino, then Tatooine again, AND then Geonosis. You are in trouble already with five planets all trying to get a full bite of the story; ESB showed that two plotlines could be done but not with this kind of attempt at emotional detail. Just about everyone agrees the Naboo scenes were a total waste of time (and a look at the cut scenes shows that GL already junked a load of them out the window, so the original plan had them even longer). AOTC really is the film that feels you are having to rush at a silly speed through things as there is just too much, and meanwhile you beg to be able to get back to Obi-Wan's interesting plotline rather than watching Anakin force-feed Padme with crap cgi fruit slices.
Thank God they blew a load of crap up at the end, even though Mace and the Jedi seem to turn up about 30 minutes ahead of their massive army simply so they could get a lot of Jedi killed and be dramatically rescued. Oh well.
Then Revenge of the Sith... which actually I think does rather well, with only Kashyyk being literally surplus to requirements and they didn't waste too much time there. So actually ROTS gets a big pass from me.
However, none of the PT are actively broken like ROTJ was, so they are still better than that.