DLC is just a natural evolution of expansion packs IMO.
Traditionally, an expansion pack had to have sufficient content to be sold at retail (i.e. priced at £9.99+ on release), whereas the advent of digital distribution has opened up the possibility for 'microtransactions' or basically minor enhancements (new characters, items etc) at a low price in addition to your more traditional expansions (new races, new campaigns/levels etc).
At the end of the day it is down to the consumer whether they choose to buy DLC or not, rather than whinging about DLC coming out on release day, take a glass half full view and think "this is fantastic, normally I have to wait 6 months for an expansion, so I can now buy this earlier than I would normally!". If you aren't happy with the amount of content in the main game as outlined in reviews, then just don't buy it. I actually think publishers are taking the right approach; release a game and then strike while the iron is hot with DLC, get players who liked your game stumping up more cash before they've moved onto something else and forgotten about your game.
The only slightly annoying thing is when you buy a combo pack that includes DLC but then more DLC comes out later (and is included in updated versions of the combo pack), effectively it means you are penalised for being an early adopter as you've now got an 'incomplete' version of the complete package (i.e. missing some DLC). Still, as I mentioned above, you were happy with the original purchase you made so can't complain too much.