Unless they plan on adding extra multiplayer goodies to the subsequent "episode", then the real driver to buy anything after the first one would have to be a nice, solid single player-experience and plot-line.
From the official Starcraft II FAQ:
How will the expansion sets impact multiplayer gameplay?
The expansion sets will add new content to each race for use in multiplayer matches. This could include additions such as new units, abilities, and structures, along with new maps and Battle.net updates.
However, they are definitely not neglecting single-player. Here's the latest news on all the work they're putting into single-player, and here's a video with a glimpse of the campaign.
1) My list was not intended to be comprehensive, it was just off the top of my head in 30 seconds. Yes, there are plenty of great 2D games out there, and still coming out today.
2) I've been "seriously" gaming since the NES (and before I saved up for my NES I played plenty of Atari 2600), so I've certainly got plenty of perspective. I still disagree with the original commenter's claims that "they" "just stop[ped] making 2D games" and that "2D games are not taken seriously."
You feel free to go ahead and keep complaining about the death of 2D games. If you don't mind, I'll just keep playing and enjoying the new ones that continue to come out. Thanks!
Same thing with 2D sprite-based games. 3D comes along, and people at first thing Great! This 3D stuff is neat, now we can have 3D and 2D games. And good thing, because entire genres of games and styles of art are built around 2D graphics. There's no way people will just stop making 2D games. But the reality is that they do. After a while we only have 3D games after all, and 2D games are not taken seriously anymore.
Gotta disagree with you on the point of 2D vs. 3D games. Just off the top of my head...
I could go on and on, but the point is that there are still plenty of great 2D games being made in recent years. 2D games most certainly are "taken seriously" (whatever that means—I mean, we are talking about games here).
"A car is just a big purse on wheels." -- Johanna Reynolds