From
https://bitcointalk.org/index.php?topic=703657.0:
Altcoin Discussion - "Discussion of cryptocurrencies other than Bitcoin. Note that discussion of how these currencies *relate to* Bitcoin may fit in other categories. ". Generic coin discussion threads or discussion about altcoin prices do not belong here.
Announcements (Altcoins) - Altcoin announcements, generic (usually announcement like) coin discussion threads. If a cryptocurrency token (e.g. token on a altcoin's blockchain) is much more similar to a new coin (new technology, actual funtional purpose, etc.) rather than a conventional investment (stocks, shares, etc.), it belongs here.
tl;dr: The "Altcoin discussions" board is meant for specialized discussion threads ("this developer is doing something shady" or "this coin's feature is going to solve this and this"), while "Announcements (Altcoins)" should be used for ANN and generic discussion threads.
Also, it's not that old ANN threads are drowning out new coins - it's just that most people couldn't give a flying fuck about another "revolutionary" token that'll probably disappear in a few months, pocketing 1$ to 100$ millions in BTC from numerous gullible investors who drop money for any coin with a shiny enough logo and an ICO.