There are a handful of mods that add to a game, but there are WAY more that just add clutter to a game and screw up sight lines, make things confusing, or just look cheap alongside stuff that already looked good on their own. Mods that clarify what a shot is without looking horrible are nice to have in there at times. Functional mods that improve play are great (air ball protectors, scoop protectors/repair, etc.).
The worst of all the mods, hands down, are the colored GI bulbs. Let's just settle this for good: COLORED BULBS ARE NOT TO BE USED FOR GI. Use white light for GI only, and save us from the horror that is the spilled Skittles across the playfield look.
Below are are before/after pics on an Addams Family. Can anybody really say that the second pic is an improvement? Looks like a color disaster.
Here is what a good TAF looks like with tasteful LEDs.