You must not read the update news. Almost every other update has a whole list of fixes.
That's why I said they're engine updates, and not content. Content and fixes are two distinct concepts.
Since L4D2's release, TF2 has seen 11 "content" updates. However, the vast majority of this has been items or 'meta' features like the inception of the online shop, the ingame training/coaching, bots, and the replay feature. Now seeing as giving Rochelle a bikini and Coach a Santa beard or reskinned weapons would probably not quell your bitching, lets exclude those. The one exception being the replay feature which I imagine some people would genuinely enjoy in L4D2, so I suppose there is that.
On the other hand, TF2 has seen 8 offical maps released since LFD2's release. Almost all of these have been single stage maps, except for Thunder Mountain which contained three stages. So ten maps altogether of varying sizes. L4D2 has seen 1 new official campaign, and 3 L4D1 campaigns rebuilt for L4D2. Altogether, 16 maps, but you can even call it 9.5 if you wish to discount each rebuilt L4D1 map as only half a map; which there is some credence to, it probably requires less work than being built from scratch.
However with that said, I would imagine substantially more work is necessary to make a L4D map "Valve official" than there is for TF2. L4D requires much greater ambience, story coherence, canon alignment, NPC scripting, encounter design, and likely more, almost all of which have no presence in TF2.
I think L4D is simply harder to develop for, as it would require more effort, and most importantly an idea consistent with the scenarios already present in L4D. TF2 you can have as many maps as you want and there's no rhyme or reason for it necessary; in L4D there's a well established story you have to work with, which makes it a lot less flexible.