Thanks for catching it, really. I actually laughed, and while kicking myself said...bright one bigfish.

Best

Simon is correct, I was being over simplistic and imprecise. From GWMT: Google supports frames and iframes to the extent that it can. Frames can cause problems for search engines because they don't correspond to the conceptual model of the web.

The content can be indexed but it will typically be from the originating url as opposed to the destination url.

The question was: How does the iframe effect my SEO? Can I just optimize the main domain for search? I want the spreadshirt shop to be found under the domain name (www.blowingminds.de)

As it stands now, from looking at www.blowingminds.de, there is nothing outside the iframe. All of the iframe urls resolve to the subdomain = blowingminds.spreadshirt.de. So, once shopping begins, a product is selected for further consideration, then you are redirected to the subdomain.

If the SEO he is speaking of is that of www.blowingminds.de, and if by I want the spreadshirt shop to be found under means that when someone searches on a search engine for it I want them to find blowingminds.de and not blowingminds.spreadshirt.de then, it is likely he is going to be disappointed.

While I agree that user experience is important, Spreadshirts appears to have developed an affilate model that utilizes an iFrame wherein they keep all the juice. They do not want blowingminds.de to become stronger than spreadshirts.de. They are fine with the subdomain gaining authority (this is my opinion).

So, it would appear what he is asking is (for practical purposes) not doable. He can either use the subdomain provided by spreadshirts:  .blowingminds.spreadshirts.de or redirect to that from an Iframe which is provided.

Good catch Simon. Helps keep us on our toes to be more precise.

Jason,

Go in and turn off your twitter, G+1, plug in and then re run the app. My guess is you will then see title tags through any moz tool. If so, you can choose a different widget or move placement. (when you deactivate the plug in make sure you clear the cache before running crawl).

Hope it helps

Malte

First content in an Iframe cannot be indexed. Your site http://www.blowingminds.de has all content inside an IFrame so no value for SEO.

With some of our sites where a product/service provider provides their info via feeds to an iframe, we build the site to have its own content, etc. and place an iframe on the page with the feed coming in. (In the US we see this a lot with real estate sites though it appears to be going away even there). If we want to optimize around something within the Iframe, we have to optimize what is outside the frame in order to do so. 
As it stands now, unless I have missed something here, your site has no chance at Domain or Page Authority.

Hope this helps

Have not, but based on the service for free, it is likely worth a try given it is more robust. With most of our sites we do not have the level of complexity so it is less of a need. Hopefully, some of the mozzers with more eCommerce will see and respond. Also, if you have a private question available, you might use that to go straight to moz and see what they could suggest.

Mon.itor.us works well and is free.

It seems you are looking for something that constantly monitors the site and simply alerts you to problems. From my point of view as an agency that has more than a few sites up, it might be overkill and I am not sure of what it would be. What we do to cover what you are listing is this: We have a pro plus moz membership and do campaign tracking with it. We can see on a weekly basis via email and daily if we just log in: 4xx, 5xx errors, dupe pg titles, missing pg titles, blocked bots, etc. as well as on page SEO issues, and general robots, rel - canon, etc.

For content checking of page changes I am at a loss, error reports as above and server downtime as below (mon.itor.us) with good result. The beauty of the SEOmoz campaign for me is that it also tracks rankings, connects to G Analytics, and provides competitive link analysis DA, PA, etc.

For the Headers you can use Screaming Frog (I just love that name and it works).

Hope that helps.