I will enjoy playing the Aztecs.
Iroquois will be an interesting challenge.
England will rule the seas.
But if I am playing for the win and I am able to choose my Civilization, India will be on my short list of civs to choose from.
A single Pop 20 city as India will be more than capable of achieving mid game is far superior than two Pop 10 cities or four Pop 5 cities.
1) The Pop 20 city only has to build a single instance of library, barracks, and so forth. Multiple cities must build multiple instances of these buildings, essentially duplicating their efforts and wasting hammers.
2) Hammers not wasted in multiple builds are available for more diverse builds, therefore it is to be expected that India's cities will be more fully developed. In other words, instead of building library and barracks yet again, India's cities will be moving on to build walls, courthouses, granaries, and so on.
3) Because India's cities will be more fully developed and because the bonuses received for buildings are additive (+ for library, + for observatory, + for research lab), India's Cities will have greater bonuses.
4) Thus, even though two Pop 10 cities, four Pop 5 cities, or a single Pop 20 city all work the same number of hexes and harvest the same raw amount of commerce and hammers, because more bonuses will be applied to those harvested by the Pop 20 city (due to the presence of more buildings), the effective yield for a Pop 20 city will be far greater.
Or to illustrate the advantage given to India from a slightly different angle:
The Glory of Rome gives a 25% bonus for building any building already present in the capitol. Let Rome have an empire of ten Pop 10 cities, while India has an empire of five Pop 20 cities. And then have both of them build a Forge in all of their cities at a proposed cost of 100 hammers each to keep things easy.
Rome's cost is 100 (for the capitol) + 75x9 (for each of 9 cities) = 775
India's cost is 100x5 (for each city) = 500
This hypothetical example yields India a production advantage of 275 hammers, which equates to that many more buildings, wonders, or military units.
I admit that this is a simplistic comparison and one could argue that Rome should have ten Pop 15 cities versus five Pop 25 cities for India (and thus, a full comparisons would have to include the greater number of hexes worked by the Roman Empire), but as a starting point, I hope this lays the groundwork for why I believe India's power is not to be underestimated.
We don't know yet if Libraries etc give percentage bonusses like they did in Civ4. They could give a fixed bonus this time around, or a combination of fixed and percentage, which would throw your math off.