Jared Spool's article "Market Maturity" [1] published in 1997 is a very good way to understand what kind of experience is required for success. He argues that success depends on market maturity which roughly follows four stages: 1. Basic deliverability war, 2. Feature parity war, 3. Productivity / "ease of use" then finally 4. Price war.
Stage 3 is generally where UX becomes a priority. For a consumer web product or startup this generally makes sense as a marker for market maturity as delivering technology and shipping features has become easy and cheap. Crafting UX is difficult but probably won't be for long. For example Twitter Bootstrap seems to be lowering the cost of boilerplate UI which in turn lowers the cost of UX in general.