They've added a very large percentage of the DLC improvements in free patches. There's still a lot (and I mean a lot) that is left out, but the fundamental gameplay works well enough.
Luckily, if you find it to be halfway decent, most of the really useful DLCs aren't all that pricey, and are half-off right now on Steam. Granted, six or seven DLCs at $4-$7 each (US, I don't have access to the European store, although I think the prices and sales are usually equivalent) adds up to a fairly decent chunk of change, but it isn't too bad, and you can cut that down if you don't want to play Muslim, Indian, Norse, or Republican rulers.
The most important DLCs are the Reapers Due (adds epidemics), Way of Life (adds a lot of extra things for your rulers to do), Sons of Abraham (adds Jews to the game, and more importantly expands the implementation of Christianity massively, adding a lot to the game), and Legacy of Rome (adds factions, retinues (standing armies), ruler ambitions).
Sword of Islam (adds Muslim rulers to the game), Rajahs of India (adds Indian rulers to the game), The Old Gods (adds Norse rulers to the game and allows an earlier start date), The Horse Lords (adds Eastern Europe and Central Asian civilization to the game, along with Mongol rulers) and The Republic (adds Republics to the game) are all pretty nice.
Monks and Mystics (adds cults), Sunset Invasion (adds a non-historical Aztec invasion as a balancer for the Mongol invasion (which mostly affects Eastern lands) against Western rulers), Conclave (puts your advisors in a council that can oppose your decisions), and Jade Dragon (adds trade with China) are more controversial.
Right now, a lot of the good DLCs, along with their cosmetic add-ons, are available in a $40 bundle.