Indeed, it took tons of creativity and effort to 'develop' Portal 2 and L4D2. Same mechanics, pretty much the same art assets, and very little new content. Portal 2 had... what a six or seven hour long single player campaign coupled with a static six or seven hour long co-op (experience is the same every time)? They added a few new hazards and tricks but it's basically Portal... just not the two hour edition.
Of course it's Portal. Should they have made a racing sim instead and called it Portal 2? Should Baldur's Gate 2 have been a first person scifi shooter? Thank God at least Dragon Age 2 had the good sense to be different.
Portal 2's campaign is much longer and more elaborate than the first game's, it has new game mechanics and includes a co-op campaign that wasn't in the first game at all. How is this not a proper sequel?
L4D2 could literally have been released as a $10 DLC for L4D and no one would have batted an eye. It added three new infected, melee weapons, and four or five new maps?
Five new
campaigns (L4D1 has four) with new survivors set in a new region, with new textures, models, sound effects and music, and new game mechanics and objectives.
L4D1 has 6 guns, L4D2 adds 9 new ones (including a grenade launcher) plus 11 melee weapons. L4D2 also adds ammo powerups, laser sights, defibrillators, adrenaline shots, and bile jars. Then there are the three new infected (or four if you count the wandering witch), and new game modes.
Yep, this sure sounds like a $10 DLC to me. Or actually $0, since this is Valve we're talking about. To qualify as a sequel it should have had at least 10 new campaigns, 50 new guns and 10 new special infected.
And their next trick is Dota 2 (which, let's be honest, is just a means of marketing the Steam platform even further and gouging morons with microtrasactions for hats, skins, new heroes, flags, etc.)! Whoa... slow down there, Valve. Don't bust a vessel pumping out these revolutionary sequels. It's the same shit Activision does with Call of Durpies but since it's Valve doing the people slurp it up.
The Call of Duty series has 15 games in total, and yearly releases. Portal and L4D have two each, and as I just demonstrated the sequels both substantially differ from the first games.