I do not have one yet but I have been researching them quite extensively so I will answer to the best of my second hand knowledge. Anyone, please feel free to correct me if I am wrong!!
1. The 650 has a camera, which does geotagging. Can you switch this off before taking a picture, so when I upload a photo of the final of a multi/mystery, the coords are not in the jpg-exif?
Unknown whether you can switch on/off in the unit however location info is quite easy to remove from the picture yourself before uploading from your computer.
2. Navstar-GPS and Glonass combined. Do I notice this in improved accuracy in the city? My 300 is often 20-30 off, resulting in looking on the wrong side of the street. Outside the city, I have often 3m accuracy, which is OK. But under foliage the 300 is also not to good. Is the 650 a big improvement? This is the main reason for getting the 650 instead of my trusted 300. :-)
Out in the open country this shouldn't be any more accurate because of GLONASS. GPS technology has likely improved the accuracy over the 300 so you will likely have some improvement there. Where you should notice a big difference is in confined spaces. In tight spots with limited sky visible, having more possible satellites to access should mean you have a better chance of getting a lock on the number of satellites you need to pinpoint your location. The other big improvement with the 6xx series is apparently the satellite lock time - it seems to be quite fast.
3. Battery lifetime: do I get my 10 hours back with the battery pack?
10 hours? No. I think they are touting 6'ish however, if you are in the car then plugging the unit in should charge the batteries. With my Montana I would go on an all day caching run in the car and end the day with my battery full because it was constantly charging in the car.
4. Cache description can now hold pictures. (I guess a GSAK export-macro will come available, did not check it yet). Can you choose which pics it will download? Are spoiler-pics (yes, I am lazy in that way :-)) useable with the screen resolution?
..no info - haven't tried exporting in GSAK with pics however you can get the latest builds here which support the Oregon 6xx series.
5. The Oregon 300 could take a drop (mine did, a few times :-)). How does the 650 look in this way? Does the screen look as if it scratched soon?
The first video I saw of the 600 was one guy tossing it to another and totally fumbling the catch. It bounces then slid across the asphalt and appeared to be fine.
6. About the touch screen: when I am searching fr a cache, I have the GPS in my hand. When getting the box (or when muggles appear), I put it in my pocket. Is there a very quick way to lock the screen? Can the second sidebutton be configured this way?
I believe the 6xx series does have a configurable button which can likely be used for this purpose though I am not certain.
7. The Oregon 650 has bluetooth. Can I connect it to my iPhone, to get updated caches (status, logs) or newly published caches (from email) into the device?
Yes!! I believe you can only access it via the Basecamp app however which, at the moment, is only available on he iPhone. Supposedly a droid version will be available soon. I'm not sure whether there is any other way it interacts with the bluetooth yet.
These units also have ANT+ capability which means they can communicate with the Garmin Tempe, Chirp, heart rate monitor, and cadence/tempo sensor.