Just to play devil's advocate, I would say that almost no city in china is relaxed by international standards. Kunming is no exception. A few places in Yunnan would fit that bill, but I wouldn't call them cities. With Kunming, I think the positives are fairly obvious and already mentioned. So here are some less rosey "cons"-perhaps opinions, perhaps facts:
-many people aren't that honest or considerate, and get angry fairly easily (though less violent and confrontational than many in dongbei).
-people are shy, and not usually outgoing.
-it is not as clean as they say.
-food is edible, but not exactly top notch.
-there is very little culture, in the sense of evolving engagements with art, music, philosophy,writing, architecture, and debate. People are interested in very little besides comfort.
-relaxation, apathy, laziness, and indifference seem to be the same thing here. If you don't think those words need naturally go together, be forewarned.
- the dialect is harsh and noisey, though if you can live with that, it does have character and rich vocabulary.
-Kunming is boring, there is not much to do. Local people sing ktv, play ma jiang, play cards, shop, drink, eat, watch tv, rinse and repeat.
-it is easy to find nice people, but just try finding nice, interesting people.
-Kunming is mostly not that pretty, not anymore anyway. Most of it is a bit ugly, And the streets are often dirty.
-If you want relaxed, clean, sunny and pretty, somewhere else in Yunnan would be better. If you want vibrant, active, interesting communities, skip Yunnan altogether.
-Kunming is not a hot place to move for many in china, with the exception of seniors who come for the mild weather and low costs (except rent), or workers from particular industries. It is more of a jumping off point or stopover connecting people to Dali, Lijiang, southeast asia, etc.
Some quantum paleontologist will now jump on and attack my comments as clueless I am sure. Happy new year to them. I have been here awhile and could summon an army of people to agree with my above comments. So take them with a grain of salt, but they are not unusual observations, despite whatever the tourist sites say.