The "troubles" I see so far for this script to be fully operational online :
1. You don't test if there is an ongoing poll or not. That means that anyone can change the mod in poll during the poll itself.
2. Anyone can vote more than once. The script should remember the name of ppl who voted. (also useful for whispering)
3. Using a symbol like "!+" and "!-" should help avoid false votes
4. There should be a time limit for the poll or at least, you should count the number of ppl on the server and make your maths from this number, instead of your arbitrary "minimum 3". This way, when at least the average+1 of ppl present in the room have voted yes, the mod would change.
Also the number of "-" vote should be tested. If you have like 25% of the ppl in the room who vote no, the poll should be dismissed right away.
5. With the system described ahead, ppl should have an "info" command that would whisper them
"Number of positive votes : X
Number of negative votes : Y
Z more votes to go"
where X is vote_y, Y the number of - votes (curr_vote-vote_y) and Z is the number of votes yet to be made before the average+1 limit. Negative votes should also be Diplayed
6. The "added vote" message should be whispered, to prevent flood kick.
I'd change the first message said by your script to "^07Type '^10votemod mod_name^07' to propose a mod for poll."
The second message should tell again what is the proposed mod: "^02Voting for " + modname
That's it for now. Nice effort, make the accurate modifications required and it should be an interesting and useful script.