You know, that brings up an interesting point I hadn't considered. I was view complex as "number of component pieces". By your definition, complex would mean "time and planning needed to manage". By this new approach, yes happiness in 5 is more involved. Happiness is now one of the top three "problems" the player has to manage, and perhaps the most important one. Interesting point.
I'd say complexity is better seen as how many viable statergies or 'interesting choices' does each 'component peice' give.
When comparing civ5 happiness with civ4 happiness you also need to look at health and maintaince, as these serve the same function - limiting expansion.
One thing i do like about civ5 is that having a small 4-5 cityempire could be an optimal way of playing for certain victory conditions. So you do have the macro decision of whether you empire is going to grow tall or wide. But in terms of overcoming the limit on expansion on civ5, well i can;
-get more luxery resources (though trade, expansion or CS)
-build happiness buildings + wonders
-happiness increasing SP
In civ4 we've got maitaince to limit horizonal growth and happiness + health to limit vertical growth. I'd agree limits on vertical growth aren't that interesting in civ4. But maintaince was far more interesting in terms of the statergies i have for dealing with it. To offset the maintiance cost of expansion i civ 4 i can;
-settle cities near high gold tiles
-whip/chop a court house
-whip/chop library and run specialists to offset having a low science slider
-found a shrine a spread relgion to fund expansion
-build GL and expand up the cost
-run a war economy, run at a gold deficeit but fund expanision by pillaging and city conquest
In civ4 just seems like i've got alot more interesting and engaging options for dealing with limits on growth.