I don't see how anyone is missing out by waiting. This game doesn't have a big multiplayer component so the idea of buying on or around day 1 to participate when the online userbase is the largest isn't present.
The single player experience is going to be exactly the same, doesn't matter if it is played at release or a year later. The only thing that changes is how much extra you have in your pocket after the purchase.
Personally, while I thought the game looked good. I didn't see a multiplayer component worthy of a $60 purchase so I am definately in the camp that is waiting. Originally $30 was my buy price but then all the DLC came out and I knew an ultimate or GOTY edition was coming so now I'm waiting for that.
There are plenly of other games to play in the meantime.
I'm not sure how you define big multiplayer component, but there is a lot going on with RDR's. It's much more fleshed out than GTA IV's was. It's also a lot of fun.
The game has been easy to find for $30 for awhile now. It's also been available for $20 and $26 that I can recall. All of the DLC was on sale for $12.50. If all you care about is the single player anyway, that Undead Nightmare pack was only $5. I'm not sure how much money is being saved by waiting, unless you're then going to wait for the GOTY pack to drop as well.