Actually, nothing. There is nothing inherently wrong with crabgrass at all. It’s not dangerous to the environment. It’s just ugly. It doesn’t make a good lawn because it doesn’t look good and it’s certainly not soft. It doesn’t feel good to walk on. In fact, the stems are thick and the leaves have rough edges, so they can be abrupt or sharp on your feet as you step on them. It’s simply not a type of grass that works well as turf. It’s a weed that people prefer to get rid of. Plus, it can dominate your lawn and simply take over everything else that’s beautiful. So while it’s not harmful to you or the environment, it can take out pretty much everything else you’ve spent your time on. It’s an opportunistic plant that will take advantage of bare or weak spots in your lawn and spread like wildfire. Before you even realize it, you’ve been dominated and it may be too late to get rid of it. It’s important to tackle your crabgrass problem before it even starts in the spring. If you let your crabgrass problem go, it will die with the first frost, turn brown, and look even worse than it did before. Not only that, but it will come back worse next year.