Wow- there's a lot here that I didn't even know were limitations - after working on the platform for a few years.
But just to add some other things...
The reason you don't have a line-by-line debugger is precisely because it's a multi-tenant platform. At least that's what SFDC says - it seems like in this age of thread-rich programming, that isn't much of an excuse, but that's apparently the reason. If you have to write code, you have "System.debug(String)" as your debugger - I remember having more sophisticated server debugging tools in Java 1.2 about 12 years ago.
Another thing I really hate about the system is version control. The Spring framework is not used for what Spring is usually used for - it's really more off a configuration tool in SFDC rather than version control. SFDC provides ZERO version-control.
You can find yourself stuck for days doing something that should seem so ridiculously easy, like, say, scheduling a SFDC report to export to a CSV file and email to a list of recipients... Well, about the easiest way to do that is create a custom object with a custom field, with a workflow rule and a Visualforce email template... and then for code you need to write a Visualforce component that streams the report data to the Visualforce email template as an attachment and you write anonymous APEX code schedule field-update of the custom object... For SFDC developers, this is almost a daily task... trying to put about five different technologies together to do tasks that seem so simple.... And this can cause management headaches and tensions too - Typically, you'd find this out after getting a suggestion to do something that doesn't work in the user-community (like someone already said), and then trying many things that, after you developed them you'd find they just don't work for some odd-ball reason - like "you can't schedule a VisualForce page", or "you can't call getContent from a schedulable context" or some other arcane reason.
There are so many, many maddening little gotcha's on the SFDC platform, that once you know WHY they're there, it makes sense... but they're still very bad limitations that keep you from doing what you need to do. Here's some of mine;
You can't get record owner information "out of the box" on pretty much any kind of record - you have to write a trigger that links the owner on create of the record to the record you're inserting. Why? Short answer because an owner can be either a "person" or a "queue", and the two are drastically different entities... Makes sense, but it can turn a project literally upside down.
Maddening security model. Example: "Manage Public Reports" permission is vastly different from "Create and Customize Reports" and that basically goes for everything on the platform... especially folders of any kind.
As mentioned, support is basically non-existent. If you are an extremely self-sufficient individual, or have a lot of SFDC resources, or have a lot of time and/or a very forgiving manager, or are in charge of a SFDC system that's working fine, you're in pretty good shape. If you are not in any of these positions, you can find yourself in deep trouble.
SFDC is a very seductive business proposition... no equipment footprint, pretty good security, fixed price, no infrastructure, AND you get web-based CRM with batchable, and schedualble processing... But as the other posters said, it is really quite a ramp-up in development learning, and if you go with consulting, I think the lowest price I've seen was $200/hour.
Salesforce tends integrate with other things years after some technologies become common-place - JSON and jquery come to mind... and if you have other common infrastructures that you want to do an integration with, like JIRA, expect to pay a lot extra, and they can be quite buggy.
And as one of the other posters mentioned, you are constantly fighting governor limits that can just drive you nuts... an attachment can NOT be > 5MB. Period. And sometimes < 3MB (if base64 encoded). Ten HTTP callouts in a class. Period. There are dozens of published governor limits, and many that are not which you will undoubtedly find and just want to run out of your office screaming.
I really, REALLY like the platform, but trust me - it can be one really cruel mistress.
But in fairness to SFDC, I'd say this: the biggest problem I find with the platform is not the platform itself, but the gargantuan expectations that almost anyone who sees the platform, but hasn't developed on it has.... and those people tend to be in positions of great authority in business organizations; marketing, sales, management, etc. Huge disconnects occur and heads roll, or are threatened to roll daily - all because there's this great platform out there with weird gotchas and thousands of people struggling daily to get their heads around why things should just work when they just don't and won't.
EDIT:
Just to add to lomaxx's comments about the MVC; In SFDC terminology, this is closely related to what's known as the "viewstate" -- aand it can be really buggy, in that what is on the VF page is not what is in the controller-class for the page. So, you have to go throught weird gyrations to synch whats on the page with what the controller is going to write to SF when you click your "save" button (or make your HTTP callout or whatever).... man, it's annoying.