How to write a strong introduction into a research paper?

This is very area specific. I'll start with the caveat that I write papers in computer science, so YMMV.

The way I think about introductions (which is not to say they are GOOD introductions) is that they tell the story of the paper in brief. Every paper has a story to tell, starting with

• Here's a fascinating question
• Here's what people have tried to do (in brief: not a full related work section, but a high level assesment)
• here's the key challenge preventing further progress
• Voila: here's our complete/partial/intermediate/awesome solution
• (additionally) and here's how it works.

The intro is typically the "hook" to read the rest of the paper, so you have to provide a birds-eye view that draws the reader in without drowning them in details.

The thing that separates a good intro from a bad one is knowing where that right level of detail is, so you're not either totally vacuous or mired in details. Getting this right is an art and depends on your field, your results, the problem, and your understanding of the target audience.

I was recently forwarded (what I think) is a guide full of excellent advice, Writing Tips for Ph. D. Students by John Cochrane. In it, Cochrane has a brief section of advice on the introduction:

The introduction should start with what you do in this paper, the major contribution. You must explain that contribution so that people can understand it. Don’t just state your conclusion: “My results show that the pecking-order theory is rejected.” Give the fact behind that result. “In a regression of x on y, controlling for z, the coefficient is q.”

The first sentence is the hardest. Do not start with philosophy, “Financial economists have long wondered if markets are efficient.” Do not start with “The finance literature has long been interested in x.” Your paper must be interesting on its own, and not just because lots of other people wasted space on the subject. Do not start with a long motivation of how important the issue is to public policy. All of this is known to writers as “clearing your throat.” It’s a waste of space. Start with your central contribution.

Three pages is a good upper limit for the introduction.

This just reiterates the point both Oldboy and Suresh made that the introduction should clearly state what the paper is about, and also some more detailed advice about avoiding generic intro. statements. (Note the upper bound is good for social science articles that may be from 20~40 pages, it should be much lower for briefer articles in different fields or journals.)

Some points to take into consideration (not an exhaustive list):

1. Correct grammar: for obvious reasons.
2. Proper literature review: many readers find annoying when the authors claim to be the first people attacking the problem of interest, while the reader is well aware of other relevant references.
3. State clearly the aims and main results in the introduction. It is frustrating when you have to read the entire paper to understand its purpose.
4. Not too long, not too short. A long introduction will make the idea of skipping this section really tempting, while a short introduction might compromise clarity or points 2 and 3.
5. Cover points of interest for different audiences. For example, try to explain the impact of the paper or the topic in terms of both theoretical and practical issues.