Can you actually determine the mAh of a battery?
Multiple things with your issue:


I would never trust a tech who recommends a Task-Killer for this problem. Disqualified 100%.
Same for the one claiming all Android phones last for only one day. That's bogus. It always depends on usage. I have one lasting 7 days (well, no SIM card inserted, not WiFi activated -- it's rather used as a stand-by computing device for some apps -- but it's a phone, and it runs longer than 10h ;), and my "standard phone" ends up at ~75-80% after 14h usage.
tech #2 and #4 sound more reasonable: any app (especially if "consuming push services") can be the cause for such a problem (see below), and monitoring battery usage is a generally good idea (that tech I would trust most from the ones you mentioned)
Different kind of usage causes different battery drain.
Sometimes processes go havoc and drain the battery quite fast. I had that "big surprise" multiple times already, last time yesterday: went down from ~80% to 5% and "emergency shutdown" within ~2h while in my pocket -- I only noticed that by the "beep" on shutdown. But this should not be a regular (daily) issue; I experienced this maybe 4-5 times in a year with my current device.


recommendation: Take a look at the battery statistics (usually found in Settings &rarr; About Phone) to determine which app(s) are causing the most battery drain. Maybe you've got some "bad egg" on your device, if this happens regularly.
second recommendation: Take a look at How to deal with (orphaned) WakeLocks? -- maybe that's what affects you. Again, this could be a mal-functioning app.

battery only lasting for ~10h is nothing rare with heavy usage. Remember these are no longer "dumb-phones", but rather "pocket-computers which also can make phone calls".


Useful resources also include:


How can I tell what is really draining my battery?
How to debug a battery issue?
How can I find out what causes “Android OS” to consume battery?
Follow the tag battery-life