Was he ever looking at a prison sentence and/or the rack and/or a sword for his activities? Or was excommunication the worst he got?
IIRC, once he called for a Council, he was a candidate for the stake.
One thing to consider was that the Catholic Church was the underpinning of Western European society at the time. Being excommunicated could result in social or even civil death.
Just checked Wikipedia. The summation of the Diet of Worms was that he was an outlaw, and was subject to being killed by anyone who pleased, without penalty to them.
40 Thaler for nail removal and door repair.
It wasn’t just him that was in trouble, any political unit (country, city-state, whatever) that was known to be harboring him could not participate in the sacraments (baptism, marriage, extreme unction, etc), a very severe punishment in a believing age.
I’ve read that this might be a historical myth. Luther circulated copies of his ninety-five theses around for people to read. But there were apparently no accounts of him nailing them to a church door until decades after the supposed fact.
John Huss preached many of the same ideas as Luther and got burnt at the stake about 100 years earlier (setting off a series of wars), so there was definitely precedent that said he was risking a gruesome death. John Wycliffe, another proto-Protestant, had managed to avoid that same fate by having sympathizers in the nobility, but many of his followers were burnt after his death (and he was later exhumed and posthumously burnt).
He was definitely in pretty deep trouble from day one, although before the Diet of Worms he probably could have avoided any serious punishment had he been willing to recant.
Luther was a pretty headstrong guy. Others have suggested that his statement at Worms of “Here I stand. I can do no other” may have been more of a resignation to his fate than a statement of defiance.
He was in theoretical trouble for sure, but Martin Luther had something going for him that Hus and Wycliffe didn’t, he basically had enough powerful princes supporting him and willing to fight the Pope that he was never in immediate danger. A lot of the Germanic princes that initially supported Luther had grown essentially tired of the Papacy.
There had been a time when Rome bound Christendom together in light of the weak states, but since then the Papacy had become more annoying than it was worth for many leaders. For years its office holder had just acted as a secular prince in almost every way, and powerful rulers of countries like Spain and France had often taken control of the office such that it just became a tool in their great power plots. The confederations of smaller Germanic states saw it as a good time to break loose from all of that and get some more power for themselves.
His main support was Frederick of Saxony. But Frederick wasn’t an unconditional supporter. Frederick was independent minded and didn’t like papal or imperial authority encroaching in his land so he defended Luther against demands that he be turned over to those authorities. But there were limits to how far he’d go in Luther’s defense. Frederick was himself a devout Catholic and a subject of the Holy Roman Emperor. Frederick was a powerful noble and the Emperor and the Pope didn’t want to alienate him if it could be avoided but they weren’t going to tolerate open rebellion.
Or conversely, Frederick might have decided he need Imperial or Papal support on some issue and Luther could have been the price asked.
And of course Frederick was mortal. He died in 1525 and his brother John and then John’s son John Frederick also protected Luther in turn. Ironically, John Frederick was much more openly in support of Luther than his father or uncle had been and this became a problem. John Frederick’s support for Luther was great enough that it finally led to a breach with Emperor Charles who declared John Frederick a rebel and placed him in captivity.
I know that, spoilsport. I mean BLASPHEMER! 
How many sheets of paper would have been needed for 95 theses?
And, are they two or three ply? Oh wait, you said theses. Nevermind.
“Diet of worms” jokes, to the left. “95 feces” remarks, form a line to your right, please. Let’s get organized around here.
On the left, a Prince:
Not where he eats, but where he is eaten. A certain convocation of politic worms are e’en at him. Your worm is your only emperor for diet.