Coincidentally, that's why I refuse to play SC2, they balanced all the fun out of the singleplayer campaign for the benefit of the multiplayer game.

I don't really understand why they need to use the same specs in the singleplayer game as in the multiplayer game.

Perhaps the saving grace of the single player campaign in SC2 then, at least in WoL, is the large number of units which are single player only. I'm thinking they were at least not changed, and luckily there are several:
http://starcraft.wikia.com/wiki/Category:StarCraft_II_Terran_campaign_units
I played WoL not long after it came out, probably before the swathe of balancing changes, and I can honestly say it was an incredibly fun and polished single player experience (meager story excluded).
The only thing I really didn't like was that you had to be logged into Battlenet to play (I couldn't get the offline mode to work) and Battlenet doesn't have a 'display me as invisible' setting like in steam. I don't know if this is still the case, but back then it drove me away from SC2 because I didn't want to have to deal with invites.

Fallout 3 (mostly due to initial lack of ammo)
Fallout: New Vegas
The Elder Scrolls: Skyrim

Mount & Blade: Warband. Once you've got around 40-50 Swadian Knights you can basically stomp any and every army of every size. You and 50 Swadian Knights for 50 Nord Housecurls and another 30-40 other troops? Ha, you might lose one or two Swadian Knights...to injury, not death. Or if you're in elite end game gear you can basically solo stomp groups of 40-50 enemies.

The Sims. Once your character is proficient in everything then well...half of the fun is gone. There's something exciting about not knowing whether or not your sim will burn down the house while trying to make popcorn!