It depends what kind of tapas bar you go to and what type of traditions it holds. Tapas traditionally used to be as simple as serving an olive on top of a bottle to keep the flies out. In time things progressed and became more exquisite.
I haven't been to Madrid so can't comment but I would highly recommend the inland cities, in particularly Granada, Arcos and Jerez de la Frontera.
The tapas bars in Granada work work on a kind of loyalty system: buy a nice glass of Rioja and you'll get a small plate of tapas, maybe a bocadillo or some tasty albondigas. Buy another drink and another and the quality of the tapas just keeps on getting better and better, culminating in a truly wonderful night out.
Having said that the majority of the bars overrun with British scum generally won't
serve good tapas, for obvious reasons.
If you find a good one, expect all sorts of delights, such as bacalao (fried fish), gambas pil pil (spicy prawns) olives, anchovies, bocadillos (small meaty baguette sandwiches), albondigas (paprika-spiced meatballs in thick smoky sauce), dry cured meats, pimientos, small rolled steaks, tomato and garlic salads, potatas bravas (delectable), the list goes on. Keep drinking, keep eating, keep smiling..