It's not something I can easily put my finger on. Maybe the scenery, which was a lot more interesting than, say, all of XII's bland sewers, underground station type things and forests. Sure it was mostly corridors, but pretty ones to look at, which draws me in as a player (why would I be drawn in by boring scenery).

Technically bad as in, if you analyse the story. I wasn't very clear there. XIII's just... makes no sense by the time you've got to the end and look back on what just happened. VIII is another good example (Ultimecia made little sense, time travel is usually a plothole in itself and it needed the "R=U" theory which I seem to recall Square had actually denied, to make sense of it). VII to some extent due to not being told very clearly, although this was fixed (with a debatable amount of retconning) in Advent Children.

I'm not a story analyser, but I've seen time and time again XII hailed as superb due to the story and characters being realistic with proper dark human flaws, like Ashe being pretty much as bad as the "bad guys", and the whole thing being a bit of a grey area anyway where there's no 100% definable "good/evil". Supposedly everything was tied together very well too. The characters are usually described as being "refreshingly subtle". Generally this all tends to be seen as "good technically" in a story from what I've seen.
Personally I couldn't stand it because I couldn't feel much of a connection with the characters, they were just this bunch of people I happened to be controlling. Worse, I constantly wanted someone to give Ashe a slap because I found her immensely spoilt, stroppy and unlikeable. And although some of it was good, I found most of the scenery boring (it didn't help that some areas were ridiculously long!)

Difficult to put into words. To each his own I guess; just because you lot hated XIII doesn't mean I have to, regardless of its flaws