Website design is highly subjective: and much of whether a design is chosen by a client depends on taste. However, conversion rate is black and white: a site either converts well or it doesn't. This article is a great summary of the golden rules to bear in mind when designing good-looking landing pages that convert effectively.
1) Design by the rule of Thirds. Divide up your screen into 9 squares and put the most important elements in the intersections of the central square.
2) Place important elements according to Fitt's Law. I.e. if you want someone to click on something make it bigger and closer to the middle of the page - the prime pixel area. Fitt's law is not linear, so making a button 10% bigger may improve things by 100%.
3) Pick the right fonts & font sizes to match brand perception (a trust signal) & readability (speed of reading, particularly on CTAs). In general, a bigger font leads to better conversion. Much like Fitt's law, small size changes are effective, so you can ensure your design stays beautiful.
4) Choose colours that invoke emotion. Again picking colours that match the brand perception is a good trust signal, but don't forget about preferences for your target personas.
One of the biggest challenges for designers is creating landing pages that not only look good, but also convert well. You can see why this would be the case—design goals and business goals are often at odds. UX and CRO (conversion rate optimization) principles don’t always align, either. There is, however, a solution: conversion-optimized design. Conversion-optimized design is where design theory meets conversion-rate optimization principles. The result is happier users and more profitable businesses.