I think each year has its own challenges. The web is constantly moving so you need to keep your knowledge fresh. For me this yes one of the more challenging aspects is the multi data source world we live in. The increase in data sources makes it much more challenging to interpret the data and make good decisions. The mobile phone traffic exploded last year and has continued in an upward path. Since this source is fairly new few companies have adjusted their technology and most of all their thinking to this channel.
I also think clean data is always a challenge. You constantly need to keep an eye on your data so it is clean. For example we have companies doing quality tests that generates a fair amount of data. You need to make sure you block those IPs and keep an eye on emerging data contamination sources.
This question is part of a five question series that are the most common issues people ask me.