How long should the piece of string be?
Forks that are too soft (either under sprung or under damped) will either ride down too low in the stroke, or blow through the stroke too fast. Operating at the end of the travel usually results in big deflections, bottoming, diving under brakes, and feeling very busy as the pace picks up.
On the other end if setup too firm you'll have issues with the fork absorbing trail trash and lower force impacts and it may deflect or push and generally beat you up.
Somewhere in the middle is where many peoples preference falls, with flavours to each side.
With all the variables involved though, the complete setup becomes very personal. What is too soft for one person in one application could be too firm (or too soft) for another. Having them tuned for yourself will better match them to your requirements, however to achieve that you'd need to be able to advise what they do now that you don't like along with some other info regarding application, your weight etc.
Some good advice given above regarding DYI setup, but all pivots around you having the tools and knowledge to do the work yourself. Springs, seals, fresh oil, and some seat time testing should get you OK results. A professional will likely get you closer to the mark in less time, for a few more dollars.
Don't forget the front and back work as a team. If you're over sprung at the back end, or have linkage/swingarm bearings that are semi seized up they can also make the front feel like it's doing work for both ends. If you decide to set the front up with heavier springs etc, it's likely the back will require similar treatment too.
