korea - busy places, horribly expensive shopping, a lot of kpop/kdrama marketing, you may strongly like/dislike food depending on preferences (hot, spicy, garlic). It's basically a city tour. one of better public transportation, but PACKED during rush hours. Manhattan wannabe without the music/art. recommended if you like busy city life - parks, shopping, nightlife, etc.
bad - not many english speakers, not many cultural/traditional buildings left.
japan - unique fashion brands, expensive as well, top notch food but dunno if you wanna try that with recent nuclear plant meltdown (and japan still hiding details). Has more old-fashioned places if you are into that. Good public transportation in terms of getting you everywhere, but confusing as hell. recommended if you are interested in cultural things - temples, tea, sushi, etc. Anything you wanna dig into, there's a place for you to learn and experience it, which differentiates it from other countries. unique street fashions.
bad - not many english speakers, food worries me, public transportation horribly confusing.
Bali - More of a get into a hotel and rest type, great foods, great ocean, no real city/town life. Tons of events within the hotel if you are into those things. Strong Hindu culture which is always fun to watch.
bad - not the place you wanna go alone. It's more focused on enjoying exotic island as is, which is better with a friend/gf.
Singapore - City that caters tourists well, best shopping, some great zoos out there, and good food districts. No real nightlife, I'd say it's more for families and couples. Some historic WW2 places are well preserved if you are into that. Best english speaking country (basically everyone speaks) among the list. Good public transportation but the country is so small you can just ride taxi everywhere.
bad - too organized. may be boring for you if you are looking for anything wild.
phuket - Great ocean, great view, TOO many tourists, fresh seafood but expensive even among the tourist-centered towns, It's OK if you like riding boats and going around watching things. Not really worth if you aren't nature admirer. If you are not, just google 'james bond island' on youtube and you can act as if you've been there. I'd say lesser Bali.
bad - other than unique looking islands and some religious buildings, everything you do can be done better somewhere else in SEA region. Good if you wanna enjoy what SEA can offer in a package in a very short period of time.
laos - not many tourists, great hotels, nature well preserved, it's one of 'small town with fking expensive buildings here and there' where you can enjoy luxury whilst watching average ppl's lives.
bad - You can't really involve yourself into the city. You are just a watcher, observing from one step aside them.
Kuala Lumpur - Lesser Singapore. Cheaper, less places to go, better nightlife but being better than Singapore doesn't mean much.
Bangkok - Some big interesting temples and buildings if you are into that, haven't been there since the riot, no idea how it is now. Was famous for sex tours, dunno if they still do that with recent events. I'd avoid it for now since I don't feel it's safe.
Opium field- somewhere up the Thailand, goes up to the border. You basically see the field and that's it. You have dozens of kids following you asking money. Do NOT go. period. Tourist trap.
In short
If you are looking for
busy city - Seoul, Tokyo, Singapore
City/tradition well mixed - Japan
Food - Tokyo, Kyoto, Bali, Singapore
Shopping - HK, Singapore
Religious culture - Indonesia/Thailand.
WW2 - Singapore
ocean activities - Bali, phuket.
Recommended if you are anything under 30. - China. It's always interesting to watch a rapidly evolving country and you learn so much from it. It also helps making any kind of investment decision in your life if you are into real estate. (most expensive places end up with similar conditions whether it be seoul, beijing, london, or whatever).
Things to worry:
Korea/Japan : Many will run away from you if you speak to them in English. overpriced compared to what other places can offer. very mild, but discrimination against non-white foreigners do exist.
Singapore/Malaysia : small shops and Taxi will try to rip you off if you act like a tourist.
Indonesia/India : Tour places are great. The average towns don't like foreigners. India showed arrogance. Indonesia showed distaste.
China - hectic. Above average crime-rate, like every rapid-developing countries.