We've covered sites like this before (Be Imba is probably the one best known), but here's another character comparison/improvement tool for you to use online. WoW Heroes is a site that will look up your character's significant info from the Armory, put it in an easy-to-read format, and then help you check out all your stats, enchants, and gear, and help you find improvements to what you're wearing. It's not as judgemental as Be Imba -- you just get the stats and the facts, not yelled at for not having gems in your sockets (though getting yelled at might be just the motivation you need to get better), but it does provide suggestions in a much gentler way, including what kinds of enchants to use and what kinds of instances to run.And one fun feature that isn't as easy to do on Be Imba is the comparison tool -- you can put in two characters' names and servers and very easily look at both at the same time, comparing item levels and/or seeing where the gear came from. Neat feature, and very easy to get up and running, no login or signup needed.
There is one drawback -- you can't see any characters in the system that are below level 70, so WoW Heroes is only for people trying to find improvements and options in the endgame -- if you're below 70, you'll have to go elsewhere. But as an endgame character improvement tool, it's another good option to use.
Recently members of my guild have been using a tool called 
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