No aspect of any Civ game can be regarded as perfectly realistic, but as someone else said even the best sims must inevitably have their inaccuracies. Of course it is possible for a game to reach a point where greater gameplay value can only be achieved at the expense of realism or vice-versa, but imo no past or present version of Civ has ever come anywhere near that point. This means there is still alot of room to make improvements which can simultaneously enhance both gameplay and realism. Imo Civ5's detractors are annoyed because Civ5 took an overall step back from realism compared to Civ4 even though the overall improvements to gameplay were questionable at best, while the previous trend from Civ1-Civ4 had generally been toward greater realism *and* better gameplay.
A number of the changes increase realism:
1. A more faithful geometry, doing away with diagonal moves being faster.
2. War is combination of battles in the field and city sieges, not just SoD in the field against SoD garrisoned in the city.
3. No more suicide catapults.
4. The "I show you mine, you show me yours" model of tech trading (seriously, when has that ever happened) has given way to a more realistic (if anachronistic) model.
5. Roads and buildings require upkeep.
6. Specialists are people working in specific buildings rather than a somewhat arbitrary number of allowed specialist. (Arbitrary, because of certain civics, wonders, and settling great people.)
7. Terrain matters more in wars.
8. The world is more than just a dozen empires.
9. Strategic resources are finite.
10. Cities don't flip because of culture.
Some of these certainly have flip sides - for instance, the battle fields are out of scale with the rest of the map until late industrialism - but it's unfair to say that the developers are just moving away from realism. It's a mixed bag; it always was.