Imagine a character called Bert. How old is he? What's his background? What clothes is he wearing? What are his shoes like? What is his relationship to Doris?
The chances are that you saw Doris and Bert as being elderly and not particularly well off (that's unless you live in Germany, where Doris is a much more up-market, younger name). They could be married to each other, or might be brother and sister. They live perhaps in council housing - they've certainly lived there for some time. Their clothes are conventional, sensible in colours like blue and grey. Doris might fancy wearing an orange scarf from time to time, but doesn't...
Names can be used as shorthand to signal things to the reader, such as age, class, nationality, education levels etc. You might want to play against that - a giant of a man known as Titch, for example - but you can go too far playing against type: if I read about this hairy hulking bloke called Rupert, I'd never be able to reconcile the conflicting images that are conjured up. My mind would snag on Rupert's name each time.
I'd also be wary of any name that was difficult to pronounce. I occasionally get to read student work featuring characters called things like Lan'Bxort. Again, my mind snags on that each time I read it. Another character whose name snags is Sir Leigh Teabing, chief baddie of The Da Vinci Code. Teabing? Who on earth is called Teabing? (It is, of course, an anagram of Baigent and Leigh, who wrote Holy Blood, Holy Grail and sued Dan Brown for plagiarism.) It's not unusual to hear people say that they find Russian novels hard because characters are called so many different names, depending on who they are with.
Watch out for characters with similar names - Ron, Dan, Don etc - which potentially could confuse a reader. I have a particular propensity for this and have to be ruthless at checking having gone in for Pat and Patrick in the same draft, along with George, John, Jenny, June and Justine. Now, when I've finished a novel, I write out an alphabet and put down each character's name beside the appropriate letter, starting with the main characters and working my way through to the minor ones. This ensures that no two characters have similar names. It also means that minor characters get randomly assigned names that I don't remember - at an event this summer, someone asked me about a character called Crystal and I hadn't a clue who they meant.
For practical reasons, I'd also suggest avoiding long names (a bore to keep typing out - though useful if you're running short on the word count and have a character called Mary Jo Barlow Smythe) and names ending with S, because they can get tiresome when you have to add a possessive apostrophe, or pluralise them - the Davises stole Cerys's hat.
Some writers make the names a particular feature. JK Rowling does brilliant character names, such as Rita Skeeter for the dodgy journalist, Gilderoy Lockhart for the self-admiring teacher, Crabbe and Goyle for the schoolboy thugs. Scarlett O'Hara, from Margaret Mitchell's Gone with the Wind, sounds both feisty and special from the start, in a way that Pansy O'Hara - her original name - would not have. And of course, Dickens was famous for his names, from Sir Leicester and Honoria Dedlock to Uriah Heep and Wackford Squeers.
But it depends on your writing, and your writing style which names you choose. In general, make them character appropriate and easy to write and say.