I think if you've been keeping iOS up to date, you won't see tons of significant differences right away. What you will see, though, is that the screen is clearer and brighter, the battery will last considerably longer, the photos you take will be much better quality--especially for video. If you're a mobile gamer, you'll obviously see an improvement in the smoothness of the games you play.

+1 for Apollo for Reddit.

Overlook for weather.

Timescroller to plan conference calls with other time zones.

Squirrelit for tickets and travel stuff.

Citymapper for public transportation.

Also, what do you use your phone for? I have iConnectHue for controlling my lights, Duolinguo for studying language, Paprika for collecting and organizing recipes and Logic Remote for music composition. I'm sure, depending on your interests, lots of folks can recommend good apps.