I have completed base stuff of FFIV and advanced content. As first thing let me say that additional Advanced content is better than FF I, FFV and definitely FFVI, on the level of FFII. You actually get a story in Advanced version and story is what I like the most about FF IV. Maybe it is SNES level graphics but if it got AAA treatment it would be one of the best stories of FF series. It is deep, unexpected and all makes sense. It reminds me of FFIX in a lot of ways :). I love when RPGs slowly reveal truth about pathetic fights of mare earth dwellers are just tiny part of bigger plot with ancient, evils fallen civilisations etc. FF IX has some of it.
Dungeons in FF IV are still mazy like in FF II but little less, without damn trap rooms.
Characters all great and well written. You can always predict how character will act, but never what will happened to them and this is how I feel it should be. On the other hand antagonists are compelling and understandable.
Battles often are without mercy and some grind is required. All characters have their class and function and some are better – I mean there are major differences in power level. I like only because there is often not much choice for picking as you get set of characters you have to use. You need to play your best and worst cards to win. No one is OP.
It is not one of the best games I have played but it is close. It does a lot of things very well.