.An up-to-date list of banned cards is available online at http://dragonhighlander.net/, under the "social" section.
Of course, EDH is a casual format so each group may use house rules or modifications of this list... but a fair amount of thought and experience goes into maintaining this list, so new groups are advised start here, and it's what you can expect if you travel to a new group.
EDH bannings are made on a case-by-case basis, usually based one of three principles:
#1: A card's power level in multiplayer EDH is signficantly in excess of both it's mana cost AND power level in other formats (due to different rules or game sizes). [Examples include Panoptic Mirror and Biorythm]
#2: A card's dollar cost is prohibitive for most players and the card usually detracts from the playing experience of everyone else in the game [The Power 8]
#3: A card or class of cards can not be consistantly interpreted by all players [Silver bordered cards]
More often than not, principle #1 (EDHBP1) boils down to a "Power:Answer-availibility" ratio. This is a spectrum, from easily dealt with by cards that are easily available, to cards which require one of a handful of cards to answer and require that they are available immediately:
A card which can be answered by many other cards, in the majority of colours, can be very powerful without causing problems. Examples include anything which can be dealt with via sorcery creature removal... many cards of this type are played by almost any deck. Cards which can only be answered by one or two colours, or a handful of cards in any given deck, are magnified by the highlander nature of EDH and must be kept under tighter control. Examples include spells which must be countered or proactively prevented from being played via Meddling Mage/Voidstone Gargoyle/Jester's Cap (Coalition Victory/Biorythm) or dealt with shortly via relatively rare cards (Panoptic Mirror) lest they win the game suddenly. An opponent's ability to have such an answer available is severely restricted by the rules of EDH, so the rules must also regulate the existance of such threats.
NB: neither ubiquity among decks (Mind's Eye, for example), nor being irritating to some players (e.g.: Sensei's Divining Top), are held by the current rules body to be sufficient grounds for a card to be banned. Both are relevant to the discussion, but a card must frequently cause actual problems (substantial restriction on deck or game variety) for its ubiquity to be relevant. What constitutes "irritating" varies too much between players to act upon... one man's fun is another man's fury, so again the card must be causing problems for most players to be actionable.
On that note, community input and sentiment are heavily considered, but unanimity among the player base is not required for us to ban a card... there will always be people who disagree with any banning (or lack thereof).
EDH Rules Committee