Well Spidey, to me it's strictly flavor.
Force of Will Smith said:
Buyback:Wrong
Shifting colors:Wrong
Artifact Lands:Wrong
Splice:Wrong
Free spells with remove: wrong (force of will)
Free spells if you have the land: Wrong (Time spiral)
imprint: oh so wrong
copying any spell: Wrong
Buyback seems fine to me. I used to use some of the Buyback spells, but now the only ones that I ever bother to touch are Forbid and Capsize...
Shifting colors? I'm not even sure what you're talking about there. The laces are the first thing that come to mind, and they were only ever disliked because they sucked...
Artifact lands: I thought they might be mildly useful when they first came out (with Academy and the like). I guess they were a lot more than that in Standard though...
Splice: Aside from being somewhat underpowered, what's the problem?
ACC spells: What? Are they too good? I've always found them to be both useful and fun. The biggest problem is that they are unevenly distributed. Blue gets Force of Will and Misdirection (both very good), black gets Contagion and Spinning Darkness (excellent against certain archetypes), red gets Pyrokinesis and Fireblast (the former being decent creature removal and the latter being one of the finest finishing moves the game has ever seen), and green gets Bounty of the Hunt (great for stompy-ish decks) and nothing else of importance (except stuff like Vine Dryad). There have even been a few other decent ACC spells. But what does white get? Scars of the Veteran? Yeah, that's really scary...
Free spells: I've seen a lot of harping on this mistake, but it's not the mechanic that's the problem. No one complained about Priest of Gix. These spells were simply too good. Frantic Search and Time Spiral were the biggest culprits, but Palinchron, Cloud of Faeries, and even Pegegrine Drake were simply overpowered (much like other well-known cards of the same block).
Imprint: I never thought it was necessary to give it a lame keyword. It's a relatively unoriginal concept that was made simpler by defining it with a keyword...
Copying spells: I like Fork. A lot of people tried to abuse Mirari and the stupid thing never scared me. I've never tried Panoptic Mirror, but it sounds fun. The only problem I can think of here is that Mind's Desire is broken (an understatement in this case).
I don't think these were problematic. Concepts aren't usually problematic in and of themselves. It's almost always individual cards that are too good. Sometimes a concept just seems too--stupid (double strike). But, to me, the problem here is different. It's flavor.
Instants or sorceries in magic have always had a sort of identity. It's a spell that you cast using a certain type of mana or (ever since Legends) a combination of types. An instant always has colored requirements because it's a certain type (color) of magic. The same goes for enchantments. Artifacts are different, because they are machines or relics that have their own power. The creatures that are colorless are always (the ones you summon are, but obviously some colorless tokens are not) artifacts too. The idea of a colorless instant wouldn't work under this, and so far none have been created. I hope it stays that way, although I wouldn't put it past WotC to change this...