Echoes seem to rise from several sources. They're interesting to me, as a sort of window into my writing process. Note that some words don't count as echoes -- common verbs, pronouns, etc. For others, the echo factor has to do with proximity. A character name feels like an echo if its repeated twice in one sentence, but not every paragraph or two. An unusual adjective, say /vainglorious/, can be an echo if it appears twice in the same manuscript. Monotonous, repetitive sentence or paragraph structures can really point up words which might not otherwise echo so intensely either.
The key is whether the word draws attention to itself, and distracts the reader thereby.
Perseveration is when I get a word lodged in my mind while writing, and it appears several times across two or three pages. So for example, I might have /enspelled/. Which has loads of synonyms and near-synonyms. (One of the great glories of writing in English is our ridiculously large and varied vocabulary.) /enscorcelled/, /enchanted/, /caught within a glamer/, /magicked/, etc. If I'm looking over a manuscript and I see /enspelled/ more than once, unless there's a very good reason for it, I go on a scorched earth hunt for the sucker.
Crutch words are a special case of perseveration. While perseveration (nice echo, huh?) tends to drop off after a couple of pages, crutch words (or phrases) echo throughout a manuscript. In Madness of Flowers, one of my crutches was "For one", as in "For one, she's a right bastard and she'll kill us all." Almost without exception, that tag was a de-intensifier that I cut on rewriter -- it was a mental equivalent of saying /um/ in speech, a stall while I worked out what was coming next. It's been a different word or phrase in other books and stories, but I always seem to have a few of them.
Occasionally, even in this language you have words or phrases with few good synonyms or substitutions. Volcanoes erupt, for example. There aren't a lot of other verbs for that. (Feel free to diss me on this in comments, but the basic point holds true.) You can talk about lava rippling or fountains of molten rock or pyroclastic flows, but there's only so many ways to say "the volcano erupted." There's a point at which the contortions to avoid the echo become more artificial than the echo itself.
I hate that.
Then there's just good old-fashioned clunks. For example, this language doesn't do a lot with pronoun case. The sentence, "Susie gave Jane her purse." is inherently ambiguous. Whose purse did Susie give to Jane? You can write around it to some degree, but the simple fact is that native English speakers cope with these ambiguities every day with little to no confusion. Generally there is plenty of context by the time you get to a sentence like that. Likewise, the degree of repetition required of a major character name can turn into a clunk.
Everything about an echo is dependent on the situation, of course. But for me personally, it's one of the greatest offenses in my own writing, and something I go to great lengths to manage. Maybe someday I'll get it right.