MT4 is the most popular trading platform among retail traders and every forex broker offers it as demand is high. I think it is not a bad platform (depending on how you modified it), but there are things that should be improved, at least in my opinion.
One of my biggest problems is navigation between charts. I always have more than 20 charts open so I can analyze them and I really hate the horizontal scroll. I think they should think of something different. I also think a design change would not hurt at all.
What are your biggest issues with MT4?