Having worked with the web for quite some time, I have to say that web development never has been more fun than it is today. Here are my take on some of the challenges that a front-end web eveloper will face:
# Coding Javascript in a robust way. Backend software is evolved and it's relatively easy to create good, maintainable code there, using MVC and mature CMS's. With Javascript, I wouldn't say that it's as easy, since applications haven't been as Javascript-intense as they are now for more than half a decade. You need to put more effort into making sure all Javascript is working in all browsers and that new code is written and placed in a maintainable way.
# Keeping a good balance between long term and short term design desitions. Too short, and the code will break apart upon your next change. Too long, your team wont be agile enough.
# Communicate key design concepts, best practices and library usage throughout the team. With too little freedom, people will get bored and do a bad job. With too much freedom people will create clusters of good code that doesn't work well together. I don't want a cathedral nor a bazaar.
# Balancing automated testing. Test too much, you spend time building tests. Test too little, stuff will break in production.
# Balancing iteration time between major framework upgrades.
# Getting people to understand MVC, for all the obvious reasons.
# Communicating SEO concepts, both to editors and developers.
# Keeping sites easy to maintain for editors. If the site isn't easy to maintain for editors, it really doesn't matter how good your code is. A site needs to be easy and fun to update and keep current.