Opinions are divided on this. Some people think you should tuck in while the food is at its best rather than wait until everyone is ready to eat. If the host or hostess insists you start then it is okay to do so; otherwise old-fashioned waiting is the civilised option.
I have my own admission to make in this regard. If I'm a guest at a friend's house and the food looks especially inviting and no one is watching me, I'll surreptitiously help myself to a bite-sized portion (from my own plate) with my fingers ahead of time. This is not polite but it's kind of fun.
2. Chewing with an open mouth
This is gross. It's noisy, rude and visually disturbing. I can remember as a child working on the skill of chewing with a closed mouth and it does take some practice. If you have yet to master this technique, keep persevering. It's a habit well worth acquiring.
3. Cutting bread rolls
In an ideal world one does not cut bread rolls in half or slice pieces off them. Rather, we use our hands to break off bite-sized pieces of bread which we then butter (or dip in oil) before eating.
4. Brushing crumbs onto the floor
At fancy restaurants table crumbs are often collected and removed between courses. At my own home, depending on how crumbly the food was, how uncoordinated the guests were, and how long the dinner is likely to continue, I sometimes offer this service too.
Occasionally a guest will get in ahead of me and brush table crumbs onto the floor meaning I'll have no choice but to sweep or vacuum the moment they leave. For the record, deliberately putting crumbs on the floor is not helping. It is making more work. Please, put the crumbs down and move away from the crumbs.
5. Bringing unsuitable items to the table
Electronic devices, reading material, baseball caps, handbags and pets do not belong at the dinner table. Mind you, I'm a fine one to talk. When I was a teenager I'd remove my orthodontic plate at the table and put it on the tablecloth while I ate.
6. Scraping cutlery on the plate
The sound of a knife scraping across a plate or a spoon scraping around a dessert bowl is like fingernails on a blackboard to me. The occasional noise of metal on crockery is acceptable but when this is how someone actually eats it's a worry. If you have to perform twenty noisy and savage scrapes in order to get just one mouthful of food then you are doing something wrong.
7. Poorly timed bathroom visits
There are usually indications that dinner time might be approaching. Food will be being cooked. Plates and cutlery will be being mustered. Dinner seldom appears without a sustained sequence of cues that it is in the offing. Ignoring these signals and choosing to powder your nose just as the meal is served and everyone else is sitting down is thoughtless.
8. Holding a knife like a pen
It's mildly disconcerting to see someone holding a knife like a pen. It looks like they're trying to be delicate or refined. Urban Dictionary says "Holds Knife Like Pen" is "used to describe the table manners of a lower class or uncouth person trying to imitate what he believes erroneously to be the behaviour of the upper classes at table" - all of which seems a bit harsh.
My main beef with this grip is that it is surely ineffective for serious cutting endeavours which means it must be adopted just for show. It remains a mystery why anyone would want to show that they don't know how to hold a table knife correctly.