Planting areas:
It is important for soil to be prepared properly at the outset as this will give a good environment for the plants to live in and keep them healthier and more able to take up water and nutrients. It is much more difficult to improve a soil once the plants are in place.

Weeds can be restricted by laying mulch once a year. The mulch will also help to limit evaporation of water from the soil and so you do not need to water as much. The correct mulch can also help to improve the soil structure over time as worms take it down into the topsoil. I would avoid using semi-permeable membrane as it is difficult to improve the soil once it is down and it often starts to rise at the corners and can look unsightly.

Plants that require high levels of maintenance:
Avoid anything that is tender and needs bringing under cover over Winter e.g. Cannas.
Avoid plants in containers and hanging baskets that will need watering frequently.
Annuals are high maintenance in that they need replacing every year and have shallow root systems that can dry out quickly, so need watering more often.

Plants that require moderate levels of maintenance:
Most perennials need cutting back once or maybe twice a year e.g. geraniums, but otherwise they are pretty maintenance free as long as you don't use varieties that self seed or spread easily.
Some perennials need staking so avoid these if possible.

Many deciduous shrubs will benefit from a prune once a year, but are otherwise maintenance free. Often they can be renovated if they too large for their allotted space.

Most grasses usually only require cutting back once a year but they can be prolific self seeders, especially on windy sites.

Plants that required low levels of maintenance:
Most evergreen shrubs e.g. Box, are really low maintenance, once they are in, just let them grow. Some may want a trim every year but most people find this easier than the more complex pruning of a deciduous shrub. However, only a few evergreens add colour to the garden so adding some of the moderate maintenance plants can help create all year interest.

Some grasses stay evergreen and need very little attention e.g. Ophiopogons. Bamboos are also low maintenance as long as the roots of the spreading varieties are restricted.

Lawns:
Whilst lawns are relatively cheap and easy to lay, they can sometimes be high maintenance, especially in small gardens where some form of hard landscaping or low maintenance planting would make the garden easier to manage.
I find that most people find it difficult to envisage living without at least a small area of lawn - to some it is a soft green carpet on which to walk bare foot, or somewhere that children can play without the risk of hurting themselves.
However, in a small garden lawns can get worn out very quickly, and quite often get little light so they become more like patches of moss (which you may or may not like!)
Also, unless you let the lawn grow long, it will not encourage wildlife like plants can.
In terms of design I find that quite often, the requirement for a large area of lawn in the middle of a garden, restricts the opportunities for breaking up the garden and creating a more interesting design.

Hard landscaping:
The thing I find that most people get annoyed about is weed growth between butt jointed paving stones. I would always point paving stones to prevent this, if budget allows.

If gravel is laid, it should be over a semi permeable membrane to prevent weed growth.

Garden features:
Open water can be high maintenance especially if the pond is not ecologically balanced. Even self circulating water features will need a clean now and again as algae will grow on any surface where there is a thin film of water.