I'm running a fairly heavily modded setup right now. Loads of fun, but loooooooong load times. Turns don't lag too much fortunately.
UI:
Unique District Icons: gives the Unique Districts a Unique Icon
Real Great People: gives the Great People a Portrait/Drawing
Gameplay:
Euphoria: Adds a new Happiness tier, Euphoria (+5 Amenities)
Jungle Mills: build lumber mills on jungle
Quo's Rocketboots: +1 Move for all units
Machine Guns Range 2: +1 Range for Machine Guns
Policy Bonanza: Adds a smattering of policies
Modern Era Buildings: Adds Tier 4 Buildings for districts
Building Upgrades: Adds Upgrades to Buildings
Capture Great People: Great People may be captured, but lose their powers atm
Units .75: Units cost 75% Production/Gold/Faith
Build Envoys: Turn Production into Envoys (uses the Holy Site)
Slower Research Per Era: Makes later techs cost more than earlier ones. Civics too.
Improved Sewers: Gives sewers +1 Housing and no Maintenence
Sapping Engineers: Makes Military Engineers work like Siege Towers and Siege Towers need a Barracks or Stable to build
Good Goody Huts: Creates a lot more Goody Huts and ups the odds of good rewards
Incoming Trade Route Gold: Incoming Trades give you +2 Gold
More Barbarian EXP: Barbarians still give full EXP at any level
Tundra Farms: Build Farms on the snow
Gold Resource: Adds Gold as a Luxury
Great Musician - Christopher Tin: Adds an info era Music-man
Diverse Beleifs: Adds 5 Patheons to the game
City State Handicap: +10 Amenities for CS's
Passable Mountains: Lets you walk on and make cities on mountains... the AI doesn't know how to yet
Sukitrakt's Tonle Sap: A Natural Wonder mod that doesn't appear to be working... I wonder why
Sukitrakt's Mount Fuji: Same deal as Above
Ananse's Naturalia: Same as Above, but may require the Ananse Modpacks
Sukirakt's Wat Aran: A manmade wonder, new
Hidden Agenda's Expansion: Gives players and AI bonuses based upon their Hidden Agenda, new
Civs:
A lot of them. I mean a lot. Even the fictional civs.
Map Scripting:
Meteor Survival: A special Inland Sea map, where only the middle is warm enough to be useful
Tldr: If it ain't broken, it doesn't have enough features.