Specifically what I have seen with them:

1. The derailleurs don't shift reliably
2. The cables are cheap and stretch alot
3. The dropout are cheap and bend alot
4. The rims are cheap and are often out of true. Sometimes can not be trued.
5. The brakes are cheap and don't stop very well
6. The brakes sometimes have plastic components that fail making the brakes inoperable
7. The seats are cheap
8. The shifters are cheap and not very durable
9. The pedals are cheap and often have bent axles
10. The cable housing is cheap and often needs replacement within a year or two
11. The tires are cheap
12. The frames are cheap and heavy
13. The crank may be a one piece that you will not want to fix or be able to reasonably get chainrings for
14. The rear cluster may be hard to find replacements for

All of the above make the Walmart bikes not a good choice for anything more than light duty. I see plenty of them around that limp along but I know that for a little bit more you can get a much better bike. Something that will save you time, money and effort in the long run.

If you just want to cruise around the neighborhood occasionally, they can be a fine choice. If you regularly want to ride to the coffee shop, I'd think about spending a little more.