The infrastructure – garbage, water, electricity, traffic – cannot handle a tourism boost, it already can not handle the existing demand. To make Samui a top-10 paradise it needs to be focussed on so-called "quality tourists" – i.e., those that can afford to pay a higher price, but also expect high service and quality – rather than budget mass tourism, then there will be fund to bring the island up to a reasonable top destination-standard from the tourist tax. Many former paradise islands and other great tourist destinations have been ruined by mass tourism, and now their residents complain; places like Canary Islands, Venice, Amsterdam etc.
Major things to be sorted out, before Samui will appear as a Top-10 destination:
Stable supply of water and electricity.
Fix what tourists see and complains about...
Getting rid of the mountains of garbage piled up in the streets. They should be both replaced with garbage containers and more collection of garbage in the residential areas, so garbage don't need to be piled up in main roads.
Force taxis to drive on meter; it's not so much the price that is a problem, but that a meter is not used. Set a higher start fee, which correspond to the actual costs of running a taxi-business of the island; if a taxi driver don't use the meter, the ride should be free, a sign in the taxis should say that.
Public waste bins; look at Singapore as a great example.
Repair side walks – Chaweng Beach, a major tourist center, is for example awful – again Singapore is a great example of how it can be in a tourist spot, compare for example with Sentosa Island.
Make rules for beaches; so, some beaches are places with jet skies and parties; others more quiet without any jet ski and ideel for noiseless paddle boards and other swimming activities. It's not all tourists that enjoys noise.
Maintain infrastructure, all down to street light lamps working, fixing holes in walk ways and especially keeping all tourist areas clean and in good looking condition; once again, Singapore's Sentosa is an excellent example.