A few years ago, there were shows like buffy/angel/heroes and others which, although had comedic moments, were serious and spent time reflecting.
Currentlt we have Psych, No Ordinary Family, Castle which have a lighthearted take on crime dram. Dexter is an exception.

Has the audience grown tired of serious shows due to the financial crisis?

What about Supernatural, Hawaii Five-O, Terriers, Burn Notice, White Collar, The Closer or Covert Affairs to name a few? I don't even see No Ordinary Family as "light" as you seem to as the show progresses. The story arc for this season is starting to get pretty dark as it progresses.

Are you asking if shows with a strictly dramatic have lost their audience due to a lack of interest? Or if these shows are down due to real-life economic/social issues of the day?

I wouldn't be surprised if more serious minded shows are not doing as well right now due to economic stress. Many people (not all!) view television as an escape from the pressures and issues of real-life. It may be the current trend to produce dramatic procedural shows that have a more overt comedic tone.

As far as another Buffy, or Angel, or Heroes, depends if the audience exists. Buffy and Angel were never breakout series in terms of ratings (hence why they survived on low-rated networks The WB and UPN). Heroes died due to a combination of the writer's strike (which resulted in lower TV viewership across the board) and terrible storytelling after season one.

It's a cyclical business, and trends change. It may be in a few years we return back to shows that have a more serious, reflective tone. Doesn't mean TV is getting worse, it's a constantly evolving medium that has to make changes, and part of those changes involve sustaining an audience.

No. There's always been room for light hearted dramas in the mix with the super serious ones which still take up most of the air waves. However, it does appear people have grown tired of these shows taking themselves too seriously. Alas, serious drama is still the most highly rated stuff out there - NCIS, CSI, CSI:NY, Law and Order:SVU, Southland, The Closer, The First 48 (though that's reality tv), etc;

Also, Psych started at the same time as Heroes, so it isn't exactly a new breed of show. Plus it survives off comparatively low ratings since it's on cable.

The Good Wife is a serious drama with lighthearted moments that is critically acclaimed and garners high ratings.

i don't watch that due to the economic climate.

Wait, what? Southland...highly-rated? Is that why NBC canceled it and it was relegated to TNT (Is that even the network it's on now, I don't know.)

Southland may be a critical darling, and I don't even know if that much is true, but I wouldn't use it to try and make a point that "serious drama is still the most highly rated stuff out there".

It seems that once Monk took off, tons of comedy-heavy drama shows started popping up. Chuck, Psych, Royal Pains, Burn Notice, etc.

They are OK but, like Monk, they get old fairly quickly as the comedy becomes the foremost element of the shows.

at Buffy, Angel, and Heroes being mentioned as "dark" shows.

Aren't all these shows on the same network (USA), anyway?

it's on TNT, more an example of just a serious show than a highly rated one.

Part of the reason that NBC didn't proceed with the Season 2 episodes was that they no longer had a 10PM hour to put it on, as Leno was occupying Mon-Fri.

I think Angel is pretty dark. Not as dark, as, say, a Lars von Trier film, but it's no Partridge Family.