A major feature of Diablo 2 was the Magic Find stat on gear. Essentially what this did was increase your chance of finding magic items and finding higher quality items. At launch this was very problematic because there were no diminishing returns on MF and eventually you could get enough MF to completely eliminate socketed gear from the loot tables which was a very undesirable result. Pretty much everybody used a few standard pieces of gear to get their MF up to a reasonable level (Goldwrap and a 49% Tarnhelm!) but things really got silly in the various xpacs. It was possible to get over 1000% MF at the end, though the diminishing returns at that point were so crippling that nobody could defend that sort of gearing as anything but silly even if you ignore that fact that a character so geared couldn't actually beat anything. It ended up that you really wanted +200-300% MF on your gear but that more than that was usually wasteful.
Now in Diablo 3 MF is back. There are tons of arguments on the forums about whether or not this is a good thing and in general I think it is fine but not a big deal either way. The advantage to having MF on gear is that it really does give a completely separate dimension to figuring out gearsets. Tweaking gear and figuring out what sort of gear would make the absolute best set was tremendous fun for me in D2 and it was always based around 3 axes of power: MF, Survival, Power. You need Power to kill monsters, you need Survival to live long enough to kill monsters and you need MF to make killing monsters worthwhile. They all forward the main goal of getting stuff but based on character attibutes, player skill, style and party composition there are going to be all kinds of different decisions made on how to value each. This situation means that there will be many different right answers for building a gearset and that there cannot be a simple spreadsheet that tells you exactly what gear to wear. I like MF in that sense because I really enjoy working out the angles on how to maximize my gearset and I think it increases the complexity in an interesting way.
Some people have a lot of cognitive dissonance when they think about how they just look for MF gear to get more MF and aren't concerned much with Power which they see as the true end goal. Given that gear simply isn't divided into categories like MF/Not MF but rather just has a selection of attributes I don't buy that argument. As you get better gear you can figure out ways to either enhance MF, Power or Survival and all of them will grow in stuttering steps; the idea that you only look for better MF gear and everything else is static simply isn't reflective of reality. I also don't see the inherent superiority of "I want more Power on my gear so I can get gear that is even more Powerful!" over "I want more MF on my gear so I can get gear that has even more MF!".
The real trouble with MF is that it can be very abusive. In D2 you could get the person with stacked MF to get all the killing blows (or as many as possible, anyhow) to maximize loot for everyone. In D3 currently you can join a party with a pure MF set and reap the best loot rewards while contributing almost nothing to actually beating the monsters. Diablo is a game that is very much about collecting more powerful equipment for your character. MF, when implemented well, is a perfectly reasonable element in such a game. I hope Blizzard finds a good way to make sure that their system does not reward characters who hide in the back stacking MF to leech off of other players in groups and as long as they do that I think having MF in D3 is fine.