Dancer is an ircd derived from hybrid, as used on efnet, but with a large number of things changed to make it better suit the freenode centrally-maintained network. Additionally, FN runs a full services implementation, which protects channels and nicks. Please work with services rather than using bots where possible. If you have a situation which services genuinly cannot handle, please tell us. If something is genuinely lacking, it might be added/changed (assuming it is in line with FN philosophy).
Regardless of it's hybrid origins, a rather large number of things have been added to dancer. Here is a (long) list of the things which are different:
Server side flood control. Various kinds of floods can be detected and throttled by the server. Sufficiently large floods will cause a client to be dropped from the network.
& and + channels are disabled. Only # channels are now supported. & channels are inappropriate for FN, and + channels are largely ineffective. If you want to use a channel without ops, in the short term you can simply create it and deop yourself. If you want to run a long term channel without ops, you can ask the network administrators in #freenode to mark it as a permanent channel. It will then be impossible to gain ops on the channel, effectively emulating a + channel.
QUIT and PART reasons from users will be quoted with "s. This prevents a person from spoofing a server notice in their quit/part reason. These messages, as well as AWAY messages, will be stripped of all colour codes.
Various desync issues are no longer present. In particular, a channel topic will no longer be lost to netsplits.
If a user is set invisible (umode +i), then issuing a WHOIS on them will not show any channels they are on, except for ones which you are also on (effectively only showing places you can already see them). If you want to know what channels they are on, ask them!
Remote WHOIS messages are forbidden, and also pointless. These are WHOIS messages where you either give a server name and a nick, or the same nick twice. Idle times will be shown in all WHOIS messages.
If a user has identified with nickserv, this will be noted in their WHOIS.
Server routing is no longer generally known. MAP is disabled, and LINKS will only return a list of servers, not how they are connected up.
IRC operators are no longer visible. If you need assistance from network admin, go to #freenode.
[, \, ], and ^ will no longer be treated as the same characters as {, |, }, and ~ by the server. This affects channel names and nicks.
Channel names may not contain colour codes or other control character junk.
Channel mode (cmode) +c has been added. When set, colours, bold, underline, reverse video, ANSI escape sequences, flashing text, and beeps will all be filtered from channel traffic.
cmode +q is a new form of ban. These "quiets" are used like bans, but they don't prevent a person from joining the channel, they merely prevent them from speaking on it. Quiets will show up in the banlist as normal, but the ban mask will be prefixed by a %.
Channel forwarding, which can shift users between channels and set up antechamber models, and channels which never lose their ban lists, modes, or topics (even when empty) are available in special cases. If you have a really good reason to want this in your channel, contact FN administration in #freenode. For now, these features are restricted until it is fully understood what their consequences are.
Various new user modes (umodes) are available, see the users section for full details.