Final Fantasy 7, and unlike KiNgBrAdLeY7, I actually know why.
1. Too much watching and not enough actual playing. Too much of the game is spent in cutscenes and dialogue, and not enough time in actual gameplay. Similarly, the developers went overboard with summon animations; when a summon is used, the game essentially halts for 30 seconds or more. Also, there is no escape from this; cutscenes are not skipable, and while you could avoid using summons, the final boss has an attack with a summon-like animation that you can't prevent (unless you're powerful enough to kill the boss before then).
2. The gameplay is too easy and unbalanced. Why have status spells when enemies are non-threats and die too easily? (Status ailments do work; it's just that the enemies aren't dangerous and durable enough for them to be worth using.) Enemy Skill is a bit too powerful (too many skills for one materia slot, unlike FFV where you had to make a significant trade-off to equip something like Blue Magic). Cure is more powerful than it should be at low levels, while Full Cure is useless because it doesn't multi-target. Also, whose idea was it to put Knights of the Round in the game? Who thought that was balanced? (It *might* have been balanced if it required 1000 MP to cast or had a long casting time, but it doesn't.)
3. Too much of an emphasis on damaging attacks. This becomes especially apparent if you look at character's limit breaks; the higher level limit breaks of the characters you are allowed to use end-game all do the same boring thing: damage. More variety of limit breaks would help, as would not permanently removing that character from your party. (I actually think the game would have been a little better if it had allowed you to use the character from Disk 2 onward even if it would create an inconsistency with the plot; there are already enough inconsistencies already.)
4. Small party size (why only 3 characters when 4 was the standard for the series and JRPGs in general?) Also, characters don't fit specific roles; it is too easy for a character to be able to fill every role well at once. (FFV didn't have this issue; while any character can fill any role, she can't fill all of them at once.) Previous Final Fantasies (perhaps excluding FF6, which has some of the same issues, but not all) handled it much better; even FInal Fantasy 2 at least made an effort to encourage giving characters specific roles.
5. Too many mini-games. Basically, all of the sudden everything you learned about the game goes out the window when the control scheme changes totally. (This is especially an issue when playing the PC game; you learned what keys do what in normal gameplay, and you now need to relearn for each mini-game.) Some of them don't make much sense: There's a scene where you have to give someone CPR to continue, even though healing magic exists in the game and it is very likely that you have access to it at the point.
6. The game never opens up. In Final Fantasy 6, the game opens up when you get the second airship, allowing you to focus on exploration and actual gameplay without endless cutscenes. This never happens in Final Fantasy 7.
Really, Final Fantasy 7 was a disgrace to the Final Fantasy series.