Wolfgang - Fair questions. I'll try to summarize.
In general, the concerns focused on technology issues. Most notably, the lack of a physical paper trail for voting transactions in some machines was top on the list of the professional software folks I know. That is, there is no way in some of the systems to work back to track precisely how a given voter cast their ballot. This is heightened by comments of a former CEO of Diebold on how he'd "deliver" the election for the Republicans.
Some of the systems generated a paper "receipt" (which corresponded to a "transaction record" within the file containing voter activities) that could be presented to the person voting. This was to act as a certainty of how they voted - it also was to be a confirmation of the summary displayed on the screen of the voting machine for the voter to approve the action ("Yes, I really am certain this is how I voted.") These systems are incredibly expensive, and were not deployed in many places at all as a result.
Other systems used a similar process, displaying a confirmation of votes to be cast on the voter's ballot and then saving the transaction - but not generating a paper receipt. These were not as expensive and were more commonly used, however, there were also concerns that some models of this type of machine and some releases of software were open to hacking.
Rather infamously, a Computer Science professor demonstrated what it took to hack into one model in a Congressional hearing - He succeeded in something like 90 seconds. Diebold claimed that it was an older system and was not current. However, there was no satisfactory answer on how many of these "obsolete" systems would be in use on election day.
The least expensive, and least technologically cool, systems were the straight optical scan machines. The trusty fill in the oval or complete the line and feed the paper ballot into a reader. The most advanced versions will scan the ballot for errors or improper marks, or in some cases spoilt ballots, and reject the ballot to be corrected by the voter.
This system gives an absolute paper trail that can be manually verified if needed - and the individual ballots can be tracked to a specific voter. This means that the voting records, recorded by hand by the election workers, can be matched by voting location to every single vote cast.
These machines are also more difficult to hack, as most systems are not actually recording votes - but presence or absence of complete records. These transactions are then fed to a central machine to interpret and tally the results. If there is a challenge, the paper ballots will be available for the purpose of auditing the transactions recorded by the machine.
The use of the new technology was central to the unrest. That combined with varying reliability of audit processes and political comments made by a business eejit fed into the maelstrom.
The simple fact is, old-fashioned electioneering and dirty tricks still work better than any of the high-tech solutions right now. There is also enough room for plausible deniability where these will be around for a while.