Having been back in the US for a full year now, it's interesting to compare how differently academia works here compared to in the UK (where I worked for the preceding 4.5 years). I much prefer the US system, personally, but will try to offer an even-handed overview here. Others are of course welcome to contribute their own observations in the comments (or email me if they'd prefer their comment to be posted anonymously). I especially welcome any corrections if my observations aren't representative in some respects.
Firstly, advantages of the UK:
* No "up-or-out" tenure-track system means that junior academics are hired directly into permanent positions, which removes a major source of stress for some people.
* More research events: the number of reading groups, colloquia, works in progress seminars, etc. -- often several such events every week during term time -- contributes to a very active-feeling "research culture". I definitely appreciated that before having a kid. (Now I need all the time I can get for doing my own work!) Related: Junior academics are much more likely to receive research invitations in the UK than in the US, at least in my experience.
* Much shorter UK teaching terms, while highly condensed, may lead to more dedicated time available for research. (Though I think I nonetheless prefer the more relaxed & spread-out teaching loads of the US. I generally enjoy teaching, but not when I had to teach 10+ seminar groups in a single week.)
* More availability of grants you can apply for to potentially "buy out" your teaching & admin responsibilities and get more research time. Great for those who are good at writing successful grants. Otherwise, the application process (and low chances of success) can be a depressing time-sink. Grant funding is a major factor in determining REF scores (see below) which can lead departments to put significant pressure on academic staff to spend more time pursuing grant applications.
Disadvantages of the UK:
* Far, far less workplace autonomy. It's really impossible to exaggerate how different the jobs feel in this respect. Teaching is a completely different experience, as assessment types for a given module are fixed and standardized at a departmental level. Grading must be anonymous and "moderated" (or second-marked) by your colleagues, which rules out participation grades, oral presentations, or pretty much anything else besides the standard methods of essays & exams. You can't offer students extensions (that goes through a formal committee that will demand documentation), punish plagiarists (another formal committee, with more documentation demands) or adjust your syllabus mid-term. Depending on your institution, you may or may not be allowed to opt out of automatic voice recordings of all your lectures (or you may need to beg permission of a colleague in a senior management role). It's all extremely rigid.
* A pervasive sense of distrust. A major reason for all of the above is that, institutionally, individual faculty members are not trusted to do a good job voluntarily. There must be constant monitoring and oversight. A big part of your job is to provide that oversight by monitoring your colleagues (e.g. second-marking) and serving on committees or in managerial roles that are empowered to make the decisions that individual faculty members (qua academic) are not allowed to make on their own.
* Paperwork. There's a lot of it. Fear of litigation is, strangely enough, far more pervasive in UK academia than in the US. Paperwork is how you prove that a decision was made by a committee, following approved procedures. Also, it's what the upper management values. And upper management calls the shots.
* Hierarchy. In the US, each faculty member is more or less "their own boss", coming together when needed to make collective decisions relevant to the department as a whole. This helps to create an egalitarian workplace ethos that I greatly value. UK academia is not like this. The "Head of Department" is authorized by the university administration to unilaterally make important decisions (possibly in consultation with select colleagues serving in certain management/administrative roles). The HoD is explicitly your boss. You're made to feel a lot more like a cog in the corporate hierarchy rather than an equal member of an academic community. At least, that was my experience. No doubt how grating one finds this in practice will depend a great deal upon the specific personalities involved. But the difference in "management structure" is something to be aware of.
One result of this hierarchy, together with the generally greater power that departmental management exerts over every aspect of your working life, is that junior academics are much more vulnerable to workplace bullying in the UK. Hostile or otherwise uncollegial behaviour can be difficult to deal with anywhere, of course, but I've personally found it easier to "bracket" such experiences when in a US context, where you're generally able to be more self-directed.
* Undergraduate recruitment. UK departments are directly responsible for maintaining their budgets by recruiting new undergraduate students. This can feel a bit mercenary, and lead to an excessive focus on "customer satisfaction". ("Somebody might complain!" is a perennial objection to proposed changes, and usually considered decisive.)
* Metric-chasing: due to the above, UK departments are obsessed with the "league table" rankings that influence student enrollments. These in turn are largely determined by student satisfaction scores (hence the customer satisfaction focus) and the REF (Research Excellence Framework), which is too complicated to describe here but provides a (highly imperfect) way for administrators to quantify a department's "research output". There's a lot of pressure on departments to produce good REF scores. REF preparation typically includes attempts within a department to rate colleagues' published work (on a 4-point scale), which can be awkward.
Conclusion: I find it pretty hard to imagine a better job than that of a (tenured or tenure-track) academic in the US. Sure, grading can be a pain, but overall you have a huge amount of autonomy to research and teach about the topics you find most interesting, in whatever way you judge best. Before moving there, I hadn't realized that working conditions would be so different for academics in the UK. (Americans considering UK job offers may also find this old discussion thread on Leiter's blog eye-opening.) I'd be curious to hear more from academics in other countries (esp. Australia and Canada) whether their workplaces more closely resemble the US or the UK model -- or something completely different?