One of the main problems is that in most countries voting is anonymous.
It's hard to find a way that people can vote online, without authentication (because anyone can only vote 1 time) and assure that the vote is not bound to an identity.
Hacking is also a major drawback, as regular voting is highly decentralised in most countries.
That makes it hard to manipulate on a large scale (especially from outsiders.
i.e. not the government itself).If there is confirmed fraud on an online system (for example, more votes than citizens).
All votes are invalid and should be redone.
In regular voting only the affected district should be redone.
Also, it's nearly impossible to check if someone isn't forced to vote for someone.
In most countries, you're forced to vote alone in some kind of box where you have privacy.
Nobody can force you to vote for a candidate or check what you have done in the box.
There could be someone with a gun on your head when you vote on your computer at home.
politics.stackexchange.com/users/19365/Mathias Maes
The main two problems are security-related.
Auditability Auditability (that is, the ability to recount votes when results get challenged) is the most important problem by far for elections that matter.
With physical ballots, you can always recount the results; and, in theory at least, the ballots get stored for years.
With electronic votes, any amount of tampering could theoretically happen to the votes.
Or, as happened recently in the US, the votes could get deleted .
You can use techs like blockchain to reduce the risk of tampering with individual bulletins, but even that offers little protection if bulletins get tampered with wholesale - e.g. on the fly as they are cast, or by overwriting all votes before the counting.
In practice the only real protection is printing the electronic ballot and having the voter vet and physically cast it.
You can then use the electronic votes to compute the results.
And if the result gets challenged, you can always resort to counting the physical ballots manually.
Anonymity Anonymity has multiple underlying problems that relate to preventing people from voting more than once.
The latter is easy enough in theory: maintain a list of people who already voted, and prevent them from voting twice.
In practice it's less simple.
At the physical level, timestamps can get in the way of perfect anonymity if the user-related tokens themselves aren't anonymous as well - but then you also need to figure out how to securely and anonymously deliver the token.
Also, you might want to allow voters to be able to change their votes just in case.
Because they might want to correct a mistake, for instance.
Or, like in Estonia, because you might want a voter to be able to verify that what they've voted is what's registered in the system.
In theory you could use a cryptographically secure one-way hashing algorithm to guarantee anonymity here, but that still leaves the door open to tampering - e.g. by hacking voting devices so it confirms you've voted on X while actually casting a vote on Y. Note that, in practice, physical votes aren't a panacea either.
Horror stories include: Ballot stuffing, e.g. when a ballot box is left behind during a (fake) fire emergency.
Ballot theft, e.g. by intercepting votes sent by snail mail, by making a ballot box disappear, etc.
Irregular votes, e.g. when someone votes for you using a fake ID (or indeed no ID at all, in countries where there's no such a thing), or when dead voters are mysteriously casting votes.
It's arguably difficult to scale any of this when physical ballots are involved, but the point here is that physical ballots aren't a silver bullet.
Electronic voting, for all its problems, is not without merits in this sense.
As one who worked a bit in the field, I expect that what will triumph in the end is a combination of the two as hinted further up: electronic votes backed with physical ballots.
It gives you the best of both worlds: quick counts and auditability.
(It might simply take the shape of a sophisticated enough ballot counting machine.)
politics.stackexchange.com/users/15531/Denis de Bernardy
Transparency is one of the biggest arguments against online voting.
The more complicated the process by which votes are represented, the more expert knowledge you need to be able to trace the "route" necessary to transform the voters will into something countable and then into the actual result.
When you vote on paper you can "just" count - the "route" taken is physical
, i.e. you can monitor the paper trail, it does not take much expert knowledge.
Whereas if you let people vote online, that puts more responsibility in terms of data security into the hands of the voter (which might be perceived as positive), but knowledge about how votes will be counted or how you would go about verifying that the voting process was not corrupted, requires more than just basic computer knowledge.
politics.stackexchange.com/users/2536/DrCopyPaste
Drawbacks : User identity protection.
Hacks to tamper the data.
force voting solution to drawbacks : use bitcoin's base technology
blockchain to secure the user identity.
Most importantly it will be tamper proof.
Main challenge here will be design the right architecture and flow.
for force voting allow the voting duration for say 10 days and within that duration allow user to change their vote.
politics.stackexchange.com/users/19369/Abhijit Gujar
One aspect easily overseen is that secret voting not only means "no one can observe my vote against my will", but also "no one can observe my vote even if I want to".
Mail voting aside, this is an extremely important concept of traditional votes, as it makes it nearly impossibly to buy or blackmail votes.
If someone gives me $100 for my vote, how could I prove I voted for him?
I am not allowed to share my vote with someone.
In online voting, it's more like mail voting, which can only be tolerated for that reason if it's only a fraction of the votes.
politics.stackexchange.com/users/19371/Bastian J
BBC  has an article which asks if it is time for countries to adopt online voting. While, there may be many advantages of this, what are the principle drawbacks of online voting? It is possible that  hacking may counter the advantages of online voting.
Drawbacks of Online Voting
politics.stackexchange.com
<voting><electronic-voting>
politics.stackexchange.com/questions/27700
politics.stackexchange.com/users/18778/Shantanu Hebbar
One of the main problems is that in most countries voting is anonymous, so it is hard to find a way for people to vote online, without authentication (because a person can only vote once) and ensure that the vote is not linked to an identity. Auditability (the ability to recount votes when results get challenged) is the most important problem by far for elections that matter.  With physical ballots, you can always recount the results; and the ballots get stored for years. However, with electronic votes, any amount of tampering could theoretically happen to the votes. The main challenge here will be to design the right architecture and flow. The drawbacks are hacks, architecture and flow, transparency, user identity, auditability and tampering. 
The drawbacks are hacks, architecture and flow, transparency, user identity, auditability and tampering. 