The first is that you will start receiving spam comments, nominally responding to what you wrote but in practice containing a link, or links, to some questionable site. I assume that somebody has created a software programme that trawls the net looking for places to post promotional material thinly camouflaged as an expression of interest or support. Clicking on the link provided is always bad idea: unless you want to give your virus or malware checker some work to do!
The second, albeit marginally less annoying, thing that will happen, is that people will ask to swap links to your blog for links to their blog. On the face of it there is nothing wrong with doing this: although links between blogs with totally unrelated target audiences presumably don’t generate many hits.
In practice many of the young bloggers who do this have a strange way of behaving. They could, proactively, put a link to my blog from theirs. Then they could make contact asking me to reciprocate. This approach is almost never used but would work almost every time.
They could make contact to suggest a mutual link swap and demonstrate their sincerity by offering to link to my blog first or they could make contact and say that once I had put a link on my blog they would do the same and then actually do so. Neither approach seems to get used very often.
In practice they tend to make contact to say that once I had put a link on my blog they would do the same but then “forget” to do so until reminded or they never get around to doing so even if reminded. Both approaches are commonly used but both unfailingly result in me regarding them negatively.
And the whole process makes me sigh every time!