In my opinion, the reputation system is unnecessary. If you're going to do good deeds, you should do it by your own will, and not under the influence of an automated system.
To others, however, it seems that the mass amount of green bars underneath their posts keep them going, and encourages them to do good deeds in the community, therefore making themselves look like a better person in general, and keeping the forums ship-shape as well.
To a select few, they overlook the general purpose intended in the reputation system and abuse it instead. I remember back when this feature was on the forums, people on the market would do stuff like "5k for your dragon force, and I'll +rep you too". Although this was only for a small portion of users, it spread like wildfire, and more and more people started doing this more often.
And then there's a very miniscule group of users who just go against the system and try earning themselves the dreaded red bars. Although, if they continued to do what they did, they would eventually be punished by the fantastic group of moderators in the community, so that wouldn't be much of a problem.
I wouldn't mind having the reputation back again, but I think that everyone should start from a clean slate again. If it's being brought back after 2 years or so, [most] people have obviously changed since that period in time, thus they should start out with 0 points from the start. The reputation system would be a lot more usable if there were some rules set in place.