I
There are worse things than using "I" too often, but this is still a serious flaw that causes automatic rejection. Yes, experienced editors will notice when a writer uises "I" too often, an dthis experienced editor will then reject the writing. You can't simply go through a story an pencil out every excess I. It doesn't work. It all needs rewritten completely, and editors aren't looking for stories that need complete rewrites.
For whatever reason, many new writers seem to think first person is the easiest and most natural way to write. It may be, but it's still the hardest form of writing for a new writer to do well. Third person is much easier for new writers to handle because it avoids many of the pitfalls first person presents.
Internalization and introspection can be good things, but new writers almost always go greatly overboard in these areas with first person writing. It makes for bad reading.
New writers also seem to forget that the story happens outside the character, and the world exists outside the character. Another bad thing.
Does this mean new writers should try first person writing? No, of course not. But a new writer should be aware that first person is much tougher to write well than third person, and that there are many pitfalls. New writers should be aware that Harlequin and Silhouette romance novels are never written in first person, and that fantasy novels seldom are. New writers should also be aware that many very, very good editors simply do not like first person writing, and will reject it.
Now, I love first person writing. I've sold seven novels and close to fifty short stories that were all written in first person. It's fine to write in first person, but not because it seems easier and more natural. Good first person is a much more advanced form of writing than third person, and darned few new writers come close to getting it right.