A common complaint when rolling paint on walls is roller marks. The ridges that form as paint builds up on the edge of the roller can be annoying and time consuming to smooth out. It's easy to avoid them. The trick is in choosing the right tools and paint, and using proper painting technique.
Cheap, low quality roller covers will cause paint ridges because they are not bevelled at the edges, and the core and material are poor quality. Compare very inexpensive roller covers with pricier ones. You will see that the cheap ones are chopped off straight at the ends while many of the better ones are bevelled. Also, cheap roller covers are often built around a cardboard core. When this gets wet during painting it can lose its shape, which makes it almost impossible to maintain an even pressure. You often "get what you pay for," so don't get the cheapest tools.
The roller frame itself can make it difficult to avoid roller marks. Again, don't get the cheapest one because they are weak and too flexible. When you apply pressure a poor quality roller frame will flex too much and lay the paint on unevenly. When choosing a roller frame, push down on it. It should be barely flexible. If it flexes a lot when you put pressure on it, choose another.
Low quality paints do not level well because they are low in solids. What this means to you is that the paint will not flow smoothly. You will see stipple marks and ridges as you work. With better quality paint, small stipple marks and ridges flatten out smoothly after application. If you suspect the problem is with your paint, mix in a paint conditioner like Floetrol, which will make it flow on much nicer.
The right painting technique helps too. Keep your roller well loaded with paint. On an average smooth or lightly textured wall you'll want to work in roughly three or four foot square sections. With the loaded roller, make an "M" or "W" to lay the paint on, then roll back across it in even, smooth strokes. Work methodically from top to bottom of the wall.
Consider 100% lambswool rollers. Although they look "too fuzzy," they hold a ton of paint and lay it on very smoothly.
Don't add water to your paint thinking it will help. This will make it even worse.