There are three main issues.
- Credibility of source.
Firstly, with what intent is Russia willing to offer damaging information on Hillary Clinton to the Trump campaign? It suggests that Russia are actively trying to influence the presidential election of a foreign country, and by agreeing to meet and receive said information, the Trump campaign essentially agrees to help this foreign country in their efforts to influence the US election. This can be interpreted as treason. It doesn't matter that the Trump campaign have an independent reason to want that information .... by agreeing to take it from Russia specifically, they are essentially agreeing to help a foreign country influence national affairs, which is illegal.
Secondly, what credibility does this information have, given that it comes from Russia? We have already established that Russia are actively attempting to influence the election, so can information received from a Russian source be deemed trustworthy? Could the information have been tampered with, artificially modified to make Hillary Clinton look as bad as possible? A responsible campaign uses trustworthy sources. Russia certainly is not a trustworthy source.
Finally, there's the issue of spying. How did Russia get some highly secret, damaging information on Hillary Clinton? A reasonable guess is that the information was achieved by illegal methods, such as spying. In that case, by agreeing to accept that information, the Trump campaign is using illegaly obtained information by a foreign country, which, again, borders on treason.
All the above are reasons why colluding with Russia this one time is wrong in and of itself. But, obviously, the reason all of this is so much of a major issue is because this is not a one-off, but rather one of many indications of certain ties between the Trump administration and a hostile, foreign nation that is always classified as a US-rival, and often even as a US-enemy.