"Ask not what you can do for your country. Ask what's for lunch." - Orson Welles
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In lieu of reading incoherent forum rants:
I believe there're 2 main reasons why many Singaporeans are upset at our immigration policy (though there's a conflation of the immigration issue and the easy work visas issue).
One is that of quantity - there are too many immigrants, and they're coming too quickly (i.e. it's not immigration per se that's the issue; water is good for health, but too much and too fast leads to water intoxication). Too much immigration, too fast: it's hard to absorb them.
The other is about breakdown - they're mostly PRCs.
Other factors are that:
1) We actively bring in immigrants (as opposed to a more natural, organic flow - foreigners who graduate from local universities are offered Permanent Residence)
2) We spend a lot of money on foreigners (for example, foreigners are given scholarships to local universities en-masse)
3) We treat foreigners better than locals (e.g. there're quite a few cases of foreigners being favoured over locals for jobs, and there're many scholarships for local universities that locals do not qualify for)
4) We accept even unskilled or lowly skilled migrants (as long as they're Chinese)
5) We have essentially no social safety net/welfare, leaving us to the vagaries of the market
6) Singapore already is ridiculously crowded (this is why countries with low population densities tend to be more open to immigration). The inadequate infrastructure (crowded public transport, expensive property) doesn't help.
7) We have no political control over immigration policy - it's easier to hate something you have no control over, nor even the illusion of control over (actually we have no political control over anything, but nevermind)
8) They exacerbate inequality on both ends of the spectrum, and if you're on the lower end you'll be hit hard. I was shocked to find out the wage levels for many unskilled jobs (and you know how expensive it is to live in Singapore).
9) Exacerbation caused by the economic downturn
10) National Slavery
Addendum:
11) Some immigrants are disdainful of Singaporeans, yet the political leadership condemns not them but Singaporeans for being xenophobic.
12) The migrants are unwilling or unable to assimilate
Although we're a nation of immigrants, you'll find that most of these factors did not apply in the 19th and early 20th centuries.
You can see these factors at play in developed countries also (e.g. small towns which are more obviously swamped by immigrants are more anti-immigration, unhappiness with immigrant cooks and hairdressers in Australia), though of course there're other layers, e.g. anger at illegal immigration (anti-immigration sentiment in the US seems to be against the illegal sort only; we don't have this here since there's so much of the legal sort) and major cultural clashes/integration issues (Muslims in Europe)