Trying to be as succinct as I can...
1. Tanking is fun, but most raid fights are not fun to tank. Tank swaps are a transparently artificial gimmick to force tanks to take turns. Instead, let the tanks coordinate between themselves who is to be MT, who is to OT, who is to tank this boss and who is to tank the next. The funnest fights to tank are the ones where tanking confers a measure of personal responsibility that goes beyond taunting at 3 stacks.
2. Gearing is a problem. There's no easy or clear way to gear up for tanking; tanking gear is role specific so a piece of tanking gear won't e.g. help you grind Timeless Isle mobs very much; tanking stats don't tangibly make you feel more powerful. The gear changes coming with WoD could help with this tremendously!
3. Abusive players are a factor, but in my view a small one. For what it's worth, before I ever tanked LFR, I would have guessed that abusive players were probably the biggest factor, but after tanking several LFRs now my perspective has changed and I would say it's the fun factor not of tanking - which is fun - but of raid design that among other things dilutes one's personal role in the raid by forcing both tanks to be equal partners in all things.