Totally agree with Jon. I was in your boat a year ago, except I had no choice but to work with SP (2010). I'm never working with that again. Search for perspective on this site and others, advice is the same: run away. And if you don't run away, make sure you allocate for 3x the normal rate and/or time, because SP makes everything 3x harder - design, development, and hardware reqs.
How about an example: typical design task - differentiate different kinds of hyperlinks. In my case I wanted usernames/authors to stand out as cyan with generic and numerous article links being some neutral color - dark gray. You can't do that, sp won't let you in any of the ootb webparts. You have to roll your own from scratch. So you either a) live with every single link being the exact same class everywhere or b) invest in dev time building the feature from scratch in javascripte/.net/asp/etc. The kicker is that the same mentality pervades the system (Jon gave the technical reason: a total lack of rational OO practices). You can't modify, you can either use ootb or go rogue. There is no middle ground.
I could go on. Taxonomy and tagging is minimally useful at best and actively wrong for (arguably) most use cases. Search is pretty much Explorer search - good for some meta information on uploaded documents and slow as hell. We built our own filter webpart in javascript to display real time results (this was apparently revolutionary - "Clicking on things on the left made stuff on the right happen! You did that with *Sharepoint?!" Accessibility and graceful degradation were not something I could afford to contemplate). The other ootb search features we left in (people search is actually not bad) caused problems from a visual standpoint: the css is so baroque that I could not make the "two" searches look the same pixel for pixel because of the way SP handles masterpages and inheritance. Speaking of masterpages: SP Designer 2010, a free product advertised as a great way to get designers involved in SP projects, used to once upon a time be FrontPage.
In conclusion: projects can cost hundreds of thousands of dollars, even a million and up, for something that could be built by a tiny team of smart jr folks and Wordpress for a fraction of the cost. The only reason to use SP is if your office lives in MS Office products, wants to version control some of them slightly less retardedly than ad-hock server share folders, and have a shared vacation calendar. Oh yea, the Silverlight org chart plugin is snazzy for users for the first 2 minutes. (There are a few others, but designers aren't part of projects like that, normally).