Wait, lets take things into perspective as you're clearly wearing some rose-tinted lenses; legalization has brought some negative aspects to CO since legalization; having been there prior to Amendment 64, it has changed the state a lot, and not all for the best.
Homelessness has risen dramatically, which leads to petty crime; try taking the bus or other public transport in Denver if you want to test this hypothesis. Try to go to the park after sunset, or around the outskirts of LoDo after dark.
Secondly, housing shortages have driven home prices/rents sky high, pricing out even locals from their areas and having to commute into town; Boulder is a good example, but Denver is catching up fast. Traffic SUCKS bad if you're going anywhere from 10-9pm, and the drivers are fucking awful; I come from a place where it hardly rains, but lived in Europe for a while, and I sear its impossible to believe how bad people are at driving in the rain/snow in CO these days, there are so many accidents now.
Lastly, because I can't go on for much longer: water shortages and the ongoing water wars with housing developer's installable avariceis having a huge impact on the agricultural landscape in the state, with inevitable consequences that won't fully come to head for another few years. Water wars are abysmal here and been in full swing for at least a decade (I first came here in 2007), and none of it has been addressed, add that to the massive influx in population, mixed with myopic shortsightednesses for quick profits and you have a recipe for disaster.
I'll be honest, I don't get why most who JUST showed up for the weed didn't end up in WA or OR, the winters there are milder the housing is more plentiful, cost of living is around the same or lower and the weed is arguably better there.
With that being said, my response to Italy's initiative: Avanti ragazzi! Italy had terrible weed when I was there, it was some seed laden thing from Spain or Morocco going for stupid prices; luckily Switzerland has much better quality, albeit far more expensive. One thing you may have, a I have first hand experience with this, is pushback from large growers; Italy is one of the major cultivators of hemp, and if the business end follows the same trends as we see in the US you will see friction between the two, despite 'supposedly' having the same goals as short-terms profits are all most people see. Cross contamination, while real, gets blown out of proportion and especially in Italy local/state governments can be bought to favour one industry over another, this is happening in CO and OR.
Sidenote: CBD and other cannabinoids are under threat in foods, you'd be wise to inform yourself about what is in the pipeline if the CDPHE and DEA have their way as its all under threat:
https://steemit.com/markets/@growhempcolorado/update-on-hemp-food-and-cbd-products-im-colorado
I do not think it possible that the legalization likely to affect all those negative aspects, are certainly influenced by other causes! If in Italy you find weeds with seeds was Albanian, in the city centers are just crap or this or poor Moroccan hash. There are mainly three deliveries in Italy as Albanian sucks, the Spanish good and the most difficult to find the Italian which is better than the Dutch. For this reason legalization would lead people to smoke at least healthier and not that crap cut or soaked with everything.
Still not as bad as non-legalization my friend, I'm from Oklahoma the city I lived in, Tulsa, was listed as one of the worlds most dangerous city in 2014 and weed is punished to the fullest extent there(some ppl get life without parole). I'm used to seeing every single thing that you mentioned and more and I agree with @stea90 I don't believe all those things are related to one amendment; granted I didn't live in CO before 64 so I really don't know. As for traffic, move somewhere smaller I live outside Pueblo rarely any traffic. Water shortage is a global issue we need to address as a species not just local state economies and housing is a whole other issue entirely because the market is still in-flux from the collapse of 2008. WA legislation makes it so hard to start that's why ppl avoid that place like a plague and OR is too lacks on regulations. CO is way better off with legalization I promise of course there's bad to every law passed but to sit there and say every problem correlates with that one law is a little out there imo especially with a major city like Denver. If you don't agree go chill in my former state for a couple years then let me know how you feel. ;-) I'll put it to you this way in comparison Pueblo is one of the most crappest city in CO; we went back to Tulsa a couple weeks ago for vacation to see family and by the time our week was done we were begging to be back in Pueblo so even with all the stuff you mentioned I still would prefer legalization and I don't actually even smoke anymore. As for the DEA, I think this hash tag is brilliant #DefundtheDEA they are the whole driving force against legalization it would already be done on a national level of it wasn't for them. Then CO would just be one of many states that sells cannabis and those problems you mentioned wouldn't be as bad if they really are a result of Amendment 64.
You can not compare the American situation with European ones, are two completely different worlds, we are talking of Italy in this post, one of the most peaceful countries in the world in terms of crimes. Yes, there is the mafia that controls drug prostitution and politicians but there little street crime, then I see no possible the hardships that you can find in the great American cities.
I know I was just posting a rebuttal to the comment but yes let's stay on topic this is about Italy and legalization there not the States.